The term “fine-tuning” is used to characterizesensitive dependences of facts or properties on the values of certainparameters. Technological devices are paradigmatic examples offine-tuning. Whether they function as intended depends sensitively onparameters that describe the shape, arrangement, and materialproperties of their constituents, e.g., the constituents’conductivity, elasticity and thermal expansion coefficient.Technological devices are the products of actual“fine-tuners”—engineers and manufacturers whodesigned and built them—but for fine-tuning in the broad senseof this article to obtain, sensitivity with respect to the values ofcertain parameters is sufficient.

Philosophical debates in which “fine-tuning” appears areoften about the universe’sfine-tuning for life:according to many physicists, the fact that the universe is able tosupport life depends delicately on various of its fundamentalcharacteristics, notably on the form of the laws of nature, on thevalues of some constants of nature, and on aspects of theuniverse’s conditions in its very early stages. Variousreactions to the universe’s fine-tuning for life have beenproposed: that it is a lucky coincidence which we have to accept as aprimitive given; that it will be avoided by future best theories offundamental physics; that the universe was created by some divinedesigner who established life-friendly conditions; and thatfine-tuning for life indicates the existence of multiple otheruniverses with conditions very different from those in our ownuniverse. Sections 1–4 of the present article review the casefor this fine-tuning for life, the reactions to it, and majorcriticisms of these reactions. Section 5 turns from fine-tuning forlife to the criterion ofnaturalness—a condition of nofine-tuning in a rather different sense which applies to theories inquantum field theory and plays a large role in contemporary particlephysics and cosmology.

• 1. Fine-Tuning for Life: the Evidence
  • 1.1 Examples from Physics
    • 1.1.1 Fine-tuned constants
    • 1.1.2 Fine-tuned conditions in the early universe
    • 1.1.3 Fine-tuned laws
  • 1.2 Are Conditions Really Fine-Tuned for Life?
  • 1.3 Fine-Tuning in Biology
• 2. Does Fine-Tuning for Life Require a Response?
  • 2.1 In Which Sense Are Life-Friendly Conditions Improbable?
  • 2.2 Does Improbable Fine-Tuning Unavoidably Call for a Response?
  • 2.3 Avoiding Fine-Tuning for Life Through New Physics?
• 3. Fine-Tuning and Design
  • 3.1 The Argument from Fine-Tuning for Design Using Probabilities
  • 3.2 The Anthropic Objection
  • 3.3 Life-Friendly Conditions as Old Evidence
  • 3.4 Can We Expect a Designer to Design?
  • 3.5 An Alternative Argument from Fine-Tuning for Design
• 4. Fine-Tuning and the Multiverse
  • 4.1 The Argument from Fine-Tuning for the Multiverse as An Inference to An Anthropic Explanation?
  • 4.2 The Argument from Fine-Tuning for the Multiverse Using Probabilities
  • 4.3 The Inverse Gambler’s Fallacy Charge
  • 4.4 Independently Motivating and Testing the Multiverse Hypothesis?
• 5. Fine-Tuning and Naturalness
  • 5.1 IntroducingNaturalness
  • 5.2 Violations of Naturalness: Examples
  • 5.3 Violations of Naturalness and Fine-Tuning for Life
• Bibliography
• Academic Tools
• Other Internet Resources
• Related Entries

1. Fine-Tuning for Life: the Evidence

1.1 Examples from Physics

Our best current theories of fundamental physics are the StandardModel of elementary particle physics and the theory of generalrelativity. The Standard Model accounts for three of the known fourfundamental forces of nature—the strong, the weak, and theelectromagnetic force—while general relativity accounts for thefourth—gravity. Arguments according to which our universe isfine-tuned for life are aimed at showing that life could not haveexisted for the vast majority of other forms of the laws of nature,other values of the constants of nature, and other conditions in thevery early universe.

The following is an—incomplete—list of suggested instancesof fine-tuning for life. (For popular overviews see Leslie 1989: ch.2, Rees 2000, Davies 2006, and Lewis & Barnes 2016; for moretechnical ones see Hogan 2000, Uzan 2011, Barnes 2012, Adams 2019 andthe contributions to Sloan et al. 2020.)

1.1.1 Fine-tuned constants

• The strength of gravity, when measured against the strength ofelectromagnetism, seems fine-tuned for life (Rees 2000: ch. 3; Uzan2011: sect. 4; Lewis & Barnes 2016: ch. 4). If gravity had beenabsent or substantially weaker, galaxies, stars and planets would nothave formed in the first place. Had it been only slightly weaker(and/or electromagnetism slightly stronger), main sequence stars suchas the sun would have been significantly colder and would not explodein supernovae, which are the main source of many heavier elements(Carr & Rees 1979). If, in contrast, gravity had been slightlystronger, stars would have formed from smaller amounts of material,which would have meant that, inasmuch as still stable, they would havebeen much smaller and more short-lived (Adams 2008; Barnes 2012: sect.4.7.1).
• The strength of the strong nuclear force, when measured againstthat of electromagnetism, seems fine-tuned for life (Rees 2000: ch. 4;Lewis & Barnes 2016: ch. 4). Had it been stronger by more thanabout \(50\,\%\), almost all hydrogen would have been burned in thevery early universe (MacDonald & Mullan 2009). Had it been weakerby a similar amount, stellar nucleosynthesis would have been much lessefficient and few, if any, elements beyond hydrogen would have formed.For the production of appreciable amounts of both carbon and oxygen instars, even much smaller deviations of the strength of the strongforce from its actual value would be fatal (Hoyle et al. 1953; Barrow& Tipler 1986: 252–253; Oberhummer et al. 2000; Barnes 2012:sect. 4.7.2).
• The difference between the masses of the two lightestquarks—the up- and down-quark—seems fine-tuned for life(Carr & Rees 1979; Hogan 2000: sect. 4; Hogan 2007; Adams 2019:sect. 2.25). Partly, the fine-tuning of these two masses obtainsrelative to the strength of the weak force (Barr & Khan 2007).Changes in the difference between them have the potential to affectthe stability properties of the proton and neutron, which are boundstates of these quarks, or lead to a much simpler and less complexuniverse where bound states of quarks other than the proton andneutron dominate. Similar effects would occur if the mass of theelectron, which is roughly ten times smaller than the mass differencebetween the down- and up-quark, would be somewhat larger in relationto that difference. There are also absolute constraints on the massesof the two lightest quarks (Adams 2019: fig. 5).
• The strength of the weak force seems to be fine-tuned for life(Carr & Rees 1979). If it were weaker by a factor of about \(10\),there would have been much more neutrons in the early universe,leading very quickly to the formation of initially deuterium andtritium and soon helium. Long-lived stars such as the sun, whichdepend on hydrogen that they can burn to helium, would not exist.Further possible consequences of altering the strength of the weakforce for the existence of life are explored by Hall et al.(2014).
• The cosmological constant characterizes the energy density\(\rho_V\) of the vacuum. On theoretical grounds, outlined inSection 5 of this article, one would expect it to be larger than its actualvalue by an immense number of magnitudes. (Depending on the specificassumptions made, the discrepancy is between \(10^{50}\) and\(10^{123}\).) However, only values of \(\rho_V\) a few order ofmagnitude larger than the actual value are compatible with theformation of galaxies (Weinberg 1987; Barnes 2012: sect. 4.6;Schellekens 2013: sect. 3). This constraint is relaxed if oneconsiders universes with different baryon-to-photon ratios anddifferent values of the number Q (discussed below), which quantifiesdensity fluctuations in the early universe (Adams 2019: sect.4.2)

1.1.2 Fine-tuned conditions in the early universe

• The global cosmic energy density \(\rho\) in the very earlyuniverse is extremely close to its so-calledcritical value\(\rho_c\). The critical value \(\rho_c\) is defined by the transitionfrom negatively curved universes (\(\rho<\rho_c\)) to flat(critical density \(\rho=\rho_c\)) to positively curved(\(\rho>\rho_c\)) universes. Had \(\rho\) not been extremely closeto \(\rho_c\) in the very early universe, life could not have existed:for slightly larger values, the universe would have recollapsedquickly and time would not have sufficed for stars to evolve; forslightly smaller values, the universe would have expanded so quicklythat stars and galaxies would have failed to condense out (Rees 2000:ch. 6; Lewis & Barnes 2016: ch. 5).
• The relative amplitude \(Q\) of density fluctuations in the earlyuniverse, known to be roughly \(2\cdot10^{-5}\), seems fine-tuned forlife (Tegmark & Rees 1998; Rees 2000: ch. 8). If \(Q\) had beensmaller by about one order of magnitude, the universe would haveremained essentially structureless since the pull of gravity would nothave sufficed to create astronomic structures like galaxies and stars.If, in contrast, \(Q\) had been significantly larger, galaxy-sizedstructures would have formed early in the history of the universe andsoon collapsed into black holes.
• The initial entropy of the universe must have been exceedinglylow. According to Penrose, universes “resembling the one inwhich we live” (2004: 343) populate only one part in\(10^{10^{123}}\) of the available phase space volume.

1.1.3 Fine-tuned laws

It has been claimed that the laws of physics are fine-tuned for lifenot only with respect to the constants that appear in them but alsowith respect to their form itself. Three of the four known fundamentalforces—gravity, the strong force, andelectromagnetism—play key roles in the organisation of complexmaterial systems. A universe in which one of these forces isabsent—and the others are present as in our ownuniverse—would most likely not give rise to life, at least notin any form that resembles life as we know it. The fundamental forcewhose existence is least clearly needed for life is the weak force(Harnik et al. 2006). A universe without any weak force but with allthe other forces of the Standard Model in place and suitably adjustedmight be habitable (Grohs et al. 2018). Further general features ofthe actual laws of nature that have been claimed to be necessary forthe existence of life are the quantization principle and the Pauliexclusion principle in quantum theory (Collins 2009: 213f.).

1.2 Are Conditions Really Fine-Tuned for Life?

Considerations according to which the laws of nature, values of theconstants, and boundary conditions of the universe are fine-tuned forlife refer to life in general, not merely human life. According tothem, a universe with different laws, constants, and boundaryconditions would almost certainly not give rise toany formof life. A common worry about such considerations is that they areill-founded due to lack of a widely accepted definition of“life”. Another worry is that we may seriouslyunderestimate life’s propensity to appear under different laws,constants, and boundary conditions because we are biased to assumethat all possible kinds of life will resemble life as we know it. Ajoint response to both worries is that, according to the fine-tuningconsiderations, universes with different laws, constants, and boundaryconditions would typically give rise to much less structure andcomplexity, which would seem to make them life-hostile, irrespectiveof how exactly one defines “life” (Lewis & Barnes2016: 255–274).

Victor Stenger (2011) is extremely critical of considerationsaccording to which the laws, constants, and boundary conditions of ouruniverse are fine-tuned. According to Stenger, the form of the laws ofnature is fixed by the reasonable—very weak—requirementthat they be “point-of-view-invariant” in that, as heclaims, the laws “will be the same in any universe where nospecial point of view is present” (p. 91). Luke Barnescriticizes this claim (2012: sect. 4.1), arguing that it relies onconfusingly identifying point-of-view-invariance with non-trivialsymmetry properties that the laws in our universe happen to exhibit.Notably, as Barnes emphasizes, neither general relativity nor theStandard Model of elementary particle physics are without conceptuallyviable, though perhaps empirically disfavoured, alternatives.

A further criticism by Stenger is that considerations according towhich the conditions in our universe are fine-tuned for life routinelyfail to consider the consequences of varying more than one parameterat a time. In response to this criticism, Barnes (2012: sect. 4.2)gives an overview of various studies such as Barr and Khan 2007 andTegmark et al. 2006 that explore the complete parameter space of(segments of) the Standard Model and arrives at the conclusion thatthe life-permitting range in multidimensional parameter space islikely very small.

Fred Adams (2019) cautions against claims that the universe isextremely fine-tuned for life. According to him, the range ofparameters for which the universe would have been habitable is quiteconsiderable. In addition, as he sees it, the universe could have beenmore, rather than less, life-friendly. Notably, if the vacuum energydensity had been smaller, the primordial fluctuations (quantified byQ) had been larger, the baryon-to-photon ratio had been larger, thestrong force had been slightly stronger, and gravity slightly weaker,there might have been more opportunities for life to develop (Adams2019: sect. 10.3). If Adams is right, our universe may just begarden-variety habitable rather than maximally life-supporting.

1.3 Fine-Tuning in Biology

Biological organisms are fine-tuned for life in the sense that theirability to solve problems of survival and reproduction dependscrucially and sensitively on specific details of their behaviour andphysiology. For example, many animals rely on their visual apparatusto spot prey, predators, or potential mates. The proper functioning oftheir visual apparatus, in turn, depends sensitively on physiologicaldetails of their eyes and brain.

Biological fine-tuning has a long tradition of being regarded asevidence for divine design (Paley 1802), but modern biology regards itas the product of Darwinian evolution, notably as driven by naturaland sexual selection. Relatively recently, some researchers haveclaimed that some specific “fine-tuned” features oforganisms cannot possibly be the outcomes of Darwinian evolutionarydevelopment alone and that interventions by some designer must beinvoked to account for them. For example, Michael Behe (1996) claimsthat the so-calledflagellum, a bacterial organ that enablesmotion, isirreducibly complex in the sense that it cannot bethe outcome of consecutive small-scale individual evolutionary steps,as they are allowed by standard, Darwinian, evolutionary theory. In asimilar vein, William Dembski (1998) argues that some evolutionarysteps hypothesized by Darwinian are so improbable that one would notrationally expect them to occur even once in a volume the size of thevisible universe. Behe and Dembski conclude that an intelligentdesigner likely intervened in the evolutionary course of events.

The overwhelming consensus in modern biology is that the challenges toDarwinian evolutionary theory brought forward by Behe, Dembski andothers can be met. According to Kenneth Miller (1999), Behe’sarguments fail to establish that there are no plausible small-stepevolutionary paths which have Behe’s allegedly“irreducibly complex” features as outcomes. For example,as Miller argues, there is in fact strong evidence for a Darwinianevolutionary history of the flagellum and its constituents (Miller1999: 147–148).

2. Does Fine-Tuning for Life Require a Response?

Many researchers believe that the fine-tuning of the universe’slaws, constants, and boundary conditions for life calls for inferringthe existence of a divine designer (seeSection 3) or a multiverse—a vast collection of universes with differinglaws, constants, and boundary conditions (seeSection 4). The inference to a divine designer or a multiverse typically rests onthe idea that, in view of the required fine-tuning, life-friendlyconditions are in some sense highlyimprobable if there isonly one, un-designed, universe. It is controversial, however, whetherthis idea can coherently be fleshed out in terms of any philosophicalaccount of probability.

2.1 In Which Sense Are Life-Friendly Conditions Improbable?

Considerations as reviewed inSection 1.1 according to which the laws, constants and boundary conditions in ouruniverse are fine-tuned for life are based on investigations ofphysical theories and their parameter spaces. It may therefore seemnatural to expect that the relevant probabilities in the light ofwhich fine-tuning for life is improbable will bephysicalprobabilities. On closer inspection, however, it is difficult to seehow this could be the case: according to the standard view of physicalpossibility, alternative physical laws and constants are physicallyimpossible by the definition of physical possibility (Colyvan et al.2005: 329). Accordingly, alternative laws and constants trivially havephysical probability zero, whereas the actual laws and constants havephysical probability one. If the laws and constants that physics hasso far determined turned out to be merely effective laws and constantsfixed by some random process in the early universe which might begoverned by more fundamental physical laws, it would start to makesense to apply the concept of physical probability to those effectivelaws and constants (Juhl 2006: 270). However, the fine-tuningconsiderations as outlined inSection 1.1 do not seem to be based on speculations about any such process, sothey do not seem to implicitly rely on the notion of physicalprobability in that sense.

Attempts to apply the notion oflogical probability tofine-tuning for life are beset with difficulties as well. Criticsargue that, from a logical point of view, arbitrary real numbers arepossible values of the constants (McGrew et al. 2001; Colyvan et al.2005). According to them, any probability measure over the realnumbers as values of the constants that differs from the uniformmeasure would be arbitrary and unmotivated. The uniform measureitself, however, assigns zero probability to any finite interval. Bythis standard, the life-permitting range, if finite, trivially hasprobability zero, which would mean that life-friendly constants arehighly improbable whether or not fine-tuning in the sense ofSection 1.1 is required for life. This conclusion seems counterintuitive, butKoperski (2005) argues that it is not as unacceptable for proponentsof the view that life-friendly conditions are improbable and require aresponse as it may initially seem.

Motivated by the difficulties that arise in attempts to apply thephysical and logical notions of probability to fine-tuning for life,contemporary accounts often appeal to an essentially epistemic notionof probability (e.g., Monton 2006; Collins 2009). According to theseapproaches, life-friendly conditions are improbable in that we wouldnot rationally expect them. An obvious problem for this view is thatlife-friendly conditions are not literally unexpected for us: as amatter of fact, we have long been aware that the conditions are rightfor life in our universe, so the epistemic probability oflife-friendly conditions appears to be trivially \(1\). As Monton(2006) highlights, to make sense of the idea that life-friendlyconditions are improbable in an epistemic sense, we must find a way ofstrategically abstracting from some of our background knowledge,notably from our knowledge that life exists, and assess theprobability of life’s existence from that perspective. (SeeSection 3.3 for further discussion.)

Views according to which life-friendly conditions are epistemicallyimprobable face the challenge to provide reasons as towhy weshould not expect life-friendly conditions from an epistemicperspective which ignores that life exists. One response to thischallenge is to point out that there is no clear systematic pattern inthe actual, life-permitting, combination of values of the constants(Donoghue 2007: sect. 8), which suggests that this combination isdisfavoured in terms of elegance and simplicity. Another response isto appeal to the criterion ofnaturalness (seeSection 5), which would lead one to expect values for at least two constants ofnature—the cosmological constant and the mass of the Higgsparticle—which differ radically from the actual ones. Neitherelegance and simplicity nor naturalness dictate any specificprobability distribution over the values of the constants, however,let alone over the form of the laws itself. But proponents of the viewthat fine-tuning for life is epistemically improbable can appeal tothese criteria to argue that life-friendly conditions will be ascribedvery low probability by any probability distribution that respectsthese criteria.

2.2 Does Improbable Fine-Tuning Call for a Response?

Even if fine-tuned conditions are improbable in some substantivesense, it might be wisest to regard them as primitive coincidenceswhich we have to accept without resorting to such speculativeresponses as divine design or a multiverse. It is indeeduncontroversial that being improbable does not by itself automaticallyamount to requiring a theoretical response. For example, any specificsequence of outcomes in a long series of coin tosses has low initialprobability (namely, \(2^{-N}\) if the coin is fair, which approacheszero as the number \(N\) of tosses increases), but one would notreasonably regard any specific sequence of outcomes as calling forsome theoretical response, e.g., a re-assessment of our initialprobability assignment. The same attitude is advocated by Gould (1983)and Carlson and Olsson (1998) with respect to fine-tuning for life.Leslie concedes that improbable events do not in general call for anexplanation, but he argues that the availability of reasonablecandidate explanations of fine-tuning for life—namely, thedesign hypothesis and the multiverse hypothesis—suggests that weshould not “dismiss it as how things just happen to be”(Leslie 1989: 10). Views similar to Leslie’s are defended by vanInwagen (1993), Bostrom (2002: 23–41), and Manson and Thrush(2003: 78–82).

Cory Juhl (2006) argues along independent lines that we should notregard fine-tuning for life as calling for a response. According toJuhl, forms of life are plausibly “causally ramified” inthat they “causally depend, for [their] existence, on a largeand diverse collection of logically independent facts” (2006:271). He argues that one would expect “causally ramified”phenomena to depend sensitively on the values of potentially relevantparameters such as, in the case of life, the values of the constantsand boundary conditions. According to him, fine-tuning for lifetherefore does not require “exotic explanations involvingsuper-Beings or super-universes” (2006: 273).

The sense in which fine-tuning for lifefails to besurprising according to Juhl differs from the sense in which itis surprising according to authors such as Leslie, vanInwagen, Bostrom, Manson and Thrush: while the latter hold thatlife-friendly conditions are rationally unexpected from an epistemicpoint of view which sets aside our knowledge that life exists, Juhlholds that—given our knowledge that life exists and iscausally ramified—it is unsurprising that life dependssensitively, for its existence, on the constants and boundaryconditions.

2.3 Avoiding Fine-Tuning for Life Through New Physics?

Biological fine-tuning for survival and reproduction, as marvellous asit often appears, is regarded as unmysterious by biologists becauseevolution as driven by natural and sexual selection can generate it(seeSection 1.3). One may hope that, similarly, future developments in fundamentalphysics will reveal principles or mechanisms which explain thelife-friendly conditions in our universe.

There are two different types of scenarios of how future developmentsin physics could realize this hope: first, physicists may hit upon aso-calledtheory of everything according to which, asenvisaged by Albert Einstein,

nature is so constituted that it is possible logically to lay downsuch strongly determined laws that within these laws only rationallycompletely determined constants occur (not constants, therefore, whosenumerical values could be changed without destroying the theory).(Einstein 1949: 63)

Einstein’s idea is that, ultimately, the laws and constants ofphysics will turn out to be dictated completely by fundamental generalprinciples. This would make considerations about alternative laws andconstants obsolete, and thereby undermine any perspective according towhich these are fine-tuned for life.

Unfortunately, developments in the last few decades have not been kindto hopes of the sort expressed by Einstein. In the eyes of manyphysicists,string theory is still the most promisingcandidate “theory of everything” in that it potentiallyoffers a unified account of all known forces of nature, includinggravity. (See Susskind 2005 for a popular introduction, Rickles 2014for a philosopher’s historical account and Dawid 2013 for arecent, favourable, methodological appraisal.) But according to ourpresent understanding of string theory, the theory has an enormousnumber of lowest energy states, orvacua, which wouldmanifest themselves at the empirical level in terms of radicallydifferent effective physical laws and different values of theconstants. These would be the laws and constants that we haveempirical access to, and so string theory would not come close touniquely determining the laws and constants in the manner envisaged byEinstein.

A second type of scenario according to which future developments inphysics may eliminate at least some fine-tuning for life would be adynamical account of the generation of life-friendlyconditions, in analogy to the Darwinian “dynamical”evolutionary account of biological fine-tuning for survival andreproduction. Inflationary cosmology (Guth 1981, 2000) is a paradigmcandidate example of such an account in that it dynamically explainswhy the total cosmic energy density \(\Omega\) in the early universeis extremely close to the so-called critical value \(\Omega_c\) (seeSection 1.1)—or, equivalently, why the overall spatial curvature of the universe isclose to zero. According to inflationary cosmology, the very earlyuniverse undergoes a period of exponential or near-exponentialexpansion (“inflation”) which effectively flattens outspace and results in near-zero post-inflation curvature, leading to atotal energy density \(\Omega\) extremely close to the criticaldensity \(\Omega_c\). Further claimed achievements of inflationarycosmology include its ability to account for the observed near-perfectisotropy of the universe and the absence of magnetic monopoles. Thestrongest empirical support for inflationary cosmology, however, isnow widely believed to come from of its apparently correct predictionsof the shape of the cosmic microwave background fluctuations (PLANCKcollaboration 2014).

Inflationary cosmology’s attractions notwithstanding, itssuggested achievements are not universally recognized. (See Steinhardt& Turok [2008] for harsh criticism by two eminent cosmologists andEarman & J. Mosterín [1999] and McCoy [2015] for criticalappraisals by philosophers.) However, even if its dynamical accountsof the flatness, isotropy, and absence of magnetic monopoles in theearly universe are correct, there is little reason to accept thatsimilar accounts will be forthcoming for many other constants,boundary conditions, or even laws of nature that seem fine-tuned forlife: whereas, notably, the critical energy density \(\Omega_c\) hasindependently specifiable dynamical properties that characterize it asa systematically distinguished value of the energy density \(\Omega\),the actual values of most other constants and parameters thatcharacterize boundary conditions are not similarly distinguished anddo not form any clear systematic pattern (Donoghue 2007: sect. 8).This makes it difficult to imagine that future physical theories willindeed reveal dynamical mechanisms which inevitably lead to thesevalues (Lewis & Barnes 2016: 181f.).

3. Fine-Tuning and Design

A classic response to the observation that the conditions in ouruniverse seem fine-tuned for life is to infer the existence of acosmic designer who created life-friendly conditions. If oneidentifies this designer with some supernatural agent or God, theinference from fine-tuning for life to the existence of a designerbecomes a version of the teleological argument. Indeed, many regardthe argument from fine-tuning for a designer as the strongest versionof the teleological argument that contemporary science affords.

3.1 The Argument from Fine-Tuning for Design Using Probabilities

Expositions of the argument from fine-tuning for design are typicallycouched in terms of probabilities (e.g., Holder 2002; Craig 2003;Swinburne 2004; Collins 2009); see also the review Manson 2009. Anelementary Bayesian formulation considers the rational impact of theobservation \(R\)—that the constants (and laws and boundaryconditions) are right for life—on our degree of beliefconcerning the design hypothesis \(D\)—that there is a cosmicdesigner. According to standard Bayesian conditioning, our posteriordegree of belief \(P^+(D)\)after taking into account \(R\)is given by our prior conditional degree of belief \(P(D\mid R)\).Analogously, our posterior \(P^+(\neg D)\) that there is no cosmicdesigner is given by our prior conditional degree of belief \(P(\negD\mid R)\). By Bayes’ theorem, the ratio between the twoposteriors is

Proponents of the argument from fine-tuning for design argue that, inview of the required fine-tuning, life-friendly conditions are highlyimprobable if there is no divine designer; see Barnes 2020 for acareful case for this claim. Thus, the conditional probability\(P(R\mid \neg D)\) should be set close to zero. In contrast, it ishighly likely according to them that the constants are right for lifeif there is indeed a designer. Thus the conditional probability\(P(R\mid D)\) should be given a value not far from \(1\).Ifa sufficiently powerful divine being exists—the ideagoes—it is only to be expected that she/he will be interested increating, or at least enabling, intelligent life, which means that wecan expect the constants to be right for life on that assumption. Thismotivates the likelihood inequality

which expresses that life-friendly conditions confirm the designerhypothesis and which likelihoodists such as Sober (2003) regard as atthe core of the argument from fine-tuning for design.

Bayesians focus not only on likelihoods but also on priors andposteriors, and in their eyes the crucial significance of theinequality \(\eqref{likelihoods}\) is that it leads to a ratio\(P^+(D)/P^+(\neg D)\) of posteriors that is much larger than theratio \(P(D)/P(\neg D)\) of the priors. Whether belief in a designeris rational depends ultimately on the priors as well, but unless thosevalues dramatically favour \(\neg D\) over \(D\) in that \(P(\negD)\gg P(D)\), the posteriors will favour design in that \(P^+(D)>P^+(\neg D)\). Bayesian proponents of the argument from fine-tuningfor design conclude that our degree of belief in the existence of somedivine designer should be greater than \(1/2\) in view of the factthat there is life, given the required fine-tuning.

3.2 The Anthropic Objection

We could not possibly have existed in conditions that are incompatiblewith the existence of observers. The famousweak anthropicprinciple (WAP) (Carter 1974) suggests that this apparentlytrivial point may have important consequences:

[W]e must be prepared to take account of the fact that our location inthe universe isnecessarily privileged to the extent of beingcompatible with our existence as observers. (Carter 1974: 293,emphasis due to Carter)

Our methods of empirical observation are unavoidablybiasedtowards detecting conditions which are compatible with the existenceof observers. For example, even if life-hostile places vastlyoutnumber life-friendly places in our universe, we should not besurprised to find ourselves in one of the relatively few places thatare life-friendly and seek an explanation for this finding, simplybecause—in virtue of being living organisms—we could notpossibly have found ourselves in a life-hostile place. Biases thatresult from the fact that what we observe must be compatible with theexistence of observers are referred to asobservation selectioneffects. The observation selection effects emphasized by the weakanthropic principle with respect to location in the universe areemphasized by what Carter dubs thestrong anthropic principle(SAP) with respect to the universe as a whole:

[T]he Universe (and hence the fundamental parameters on which itdepends) must be such as to admit within it the creation of observerswithin it at some stage. (Carter 1974: 294)

Carter’s formulation of the SAP has led some authors, mostinfluentially Barrow and Tipler (1986), to misinterpret it alongteleological lines and as thereby categorically different from theWAP. But, as Carter himself highlights (1983: 352), see also Leslie(1989: 135–145), the SAP is meant to highlight exactly the sametype of bias as the WAP and is literally stronger than the WAP onlywhen conjoined with a version of the multiverse hypothesis.

The so-calledanthropic objection against the argument fromfine-tuning for design argues that that argument breaks down once ourbiasedness due to the observation selection effects emphasized by theweak and strong anthropic principles is taken into account. ElliottSober (2003, 2009) advocates this objection. According to him, theargument from fine-tuning for design requires not the likelihoodinequality \(\eqref{likelihoods}\) but the much more problematic

where “OSE” stands for “observationselection effect”. Sober himself spells outOSE as“We exist, and if we exist, the constants must be right”(2003: 44). According to this interpretation, \(\eqref{ose}\) ispatently false: our existence as living organisms entails that theconstants are right for life, which means that the terms on both sidesof \(\eqref{ose}\) are trivially \(1\) and hence equal, so\(\eqref{ose}\) does not hold on Sober’s analysis.

Critics of the anthropic objection argue that Sober’s reasoningdelivers highly implausible results when transferred to examples whererational inferences are less controversial. Most famous isLeslie’s firing squad (Leslie 1989: 13f.), in which a prisonerexpects to be executed by a firing squad but, to his own surprise,finds himself alive after all the marksmen have fired and wonderswhether they intended to miss. The firing squad scenario involves anobservation selection effect because the prisoner cannot contemplatehis post-execution situation unless he somehow survives the execution.His observations, in other words, are “biased” towardsfinding himself alive (see Juhl [2007] and Kotzen [2012] for furtheruseful examples). Sober’s analysis, applied to the firing squadscenario, suggests that it would not be rational for the prisoner tosuspect that the marksmen intended to miss (unless independentevidence suggests so) because that would mean overlooking theobservation selection effect that he faces. But, as Leslie, Weisberg(2005) and Kotzen (2012) argue, this recommendation seems veryimplausible.

According to Weisberg, Sober’s analysis fails due to itsincorrect identification of the observation selection effectOSE with “We exist, and if we exist, the constants mustbe right”. Weisberg argues that the weaker, purely conditional,statement “If we exist, the constants must be right”(Weisberg 2005: 819, Weisberg’s wording differs) suffices tocapture the observation selection effect. But if we interpret“OSE” as this statement, there is no reason tosuppose that the inequality \(\eqref{ose}\) fails and the argumentfrom fine-tuning for design appears vindicated inasmuch as theanthropic objection is concerned. (See Sober 2009 for Sober’sresponse.)

To resolve the difficulty of accommodating observation selectioneffects in likelihood arguments, Kotzen (2012) suggests that bias dueto such effects be taken into account as evidence rather thanbackground information. Notably, instead of \(\eqref{ose}\) Kotzenproposes to consider

where \(I\) contains information about the observation process,including observation selection effects (Kotzen 2012: 835). Accordingto this analysis, the argument from fine-tuning for design can besaved from the anthropic objection for a variety of ways to spell outthe information \(I\) about the observation process and anthropicbias.

3.3 Life-Friendly Conditions as Old Evidence

Views according to which life-friendly conditions are improbable in anepistemic sense due to the required fine-tuning are challenged to cometo terms with the fact that, as a matter of fact, we have long knownthat our universe is life-friendly, which means that life-friendlyconditions are notliterally unexpected for us. As aconsequence of this fact, the Bayesian version of the argument fromfine-tuning for a designer as outlined inSection 3.1 must adopt some solution to Bayesianism’s notoriousproblemof old evidence (Glymour 1980) because \(R\)—that theconstants are right for life—is inevitablyold evidencefor us.

An obvious choice, endorsed by Monton (2006), who is critical of theargument from fine-tuning for design, and Collins (2009), who supportsit, is the so-called counterfactual orur-probabilitysolution to the problem of old evidence, as defended by Howson (1991).The main advantage of this solution as applied to the argument fromfine-tuning for design is that it allows to essentially preserve theargument, including \(\eqref{simpledes}\) and \(\eqref{likelihoods}\),with the sole refinement that one must consistently construe all priorprobabilities \(P(\cdot)\), conditional and unconditional, as“ur-probabilities”, i.e., rational credences of somecounterfactual epistemic agent who is unaware that the constants areright for life. Somewhat bizarrely, as Monton points out (2006: 416),such an agent would have to be at least temporarily unaware of her/hisexistence (or at least her/his existence as a form of life) becauseotherwise she/he could not possibly be unaware that the conditions areright for life. Tentative suggestions concerning the backgroundknowledge that can reasonably be ascribed to such an agent aredeveloped by Monton (2006: sect. 4) and Collins (2009: sect. 4.3).

An advantage of approaching the argument from fine-tuning for designusing the ur-probability solution is that it offers proponents of theargument a clear-cut rejection of the anthropic objection: as inKotzen’s (2012) approach, the fact that we exist is treated notas background knowledge but as evidence taken into account by Bayesianconditioning. The appropriate comparison between likelihoods toconsider is thus not Sober’s \(\eqref{ose}\)—at least notunder Sober’s own interpretation of “OSE”as including “We exist”—but rather\(\eqref{likelihoods}\) or \(\eqref{ose_kotzen}\), both of which evadethe anthropic objection.

3.4 Can We Expect a Designer to Design?

The likelihood inequality \(\eqref{likelihoods}\) on which thefine-tuning argument for design rests is based on the assumption that,reasonably, \(P(R\mid \neg D)\) is very small because life-friendlyconditions are improbable if there is no designer. This assumption canbe challenged, as already discussed in2.1. But the likelihood inequality \(\eqref{likelihoods}\) also rests onthe assumption that \(P(R\mid D)\) is comparatively large, i.e., onthe view that, if there is indeed a designer, life-friendly conditionsare more to be expected than if there is no designer. This assumptioncan be challenged as well.

Reasonable assignments of \(P(R\mid D)\) depend on how exactly thedesigner hypothesis \(D\) is spelled out. According to Swinburne, themost promising candidate designer is “the God of traditionaltheism” whom he characterizes as “a being essentiallyeternal, omnipotent (in the sense that He can do anything logicallypossible), omniscient, perfectly free, and perfectly good”(2003: 107). Swinburne argues that we can be at least moderatelyconfident that the God of traditional theism, if he exists,“will bring about an orderly, spatially extended, world in whichhumans have a location” (2003: 113; note that Swinburne operateswith a generalized, non-biological concept of “humans”).Hence, according to Swinburne, life-friendly conditions, conditionalon the existence of the God of traditional theism, do not have verylow probability, i.e. \(P(R\mid D)\), plausibly, is not many orders ofmagnitude smaller than \(1\). According to Rota (2016: 119f.), even ifwe assign to \(P(R\mid D)\) a value as low as \(1\) in a billion, thissuffices for the fine-tuning argument for a divine designer to bestrong, simply because, in view of the fine-tuning considerations,life-friendly conditions in the absence of a designer are so utterlyunexpected. A similar point is made by Hawthorne and Isaacs(2018).

Criticisms of the view that life-friendly constants are to be expectedif there is a designer have a long tradition and go back to John Venn(1866) and John Maynard Keynes (1921). More recently, Sober has voicedgeneral reservations about our abilities to competently judge what adivine designer, if real, would do:

Our judgements about what counts as a sign of intelligent design mustbe based on empirical information about what designers often do andwhat they rarely do. As of now, these judgements are based on ourknowledge ofhuman intelligence. The more our hypotheses ofintelligent designers depart from the human case, the more in the darkwe are as to what the ground rules are for inferring intelligentdesign. (Sober 2003: 38)

In a similar spirit, Narveson complains that we are in no position topredict how a cosmic designer would behave because “[b]odilessminded super-creators are a category that is way, way out ofcontrol” (Narveson 2003: 99). According to Sober and Narveson itis particularly problematic for theists to confidently assume thatGod, if she/he exists, would create life-friendly conditions and, atthe same time, react to the problem of evil by highlighting ourinability to understand “the mysterious ways of the Deity”(Narveson 2003: 99). Manson (2020) provides an updated defense of theposition of Sober and Narveson against the arguments of Rota (2016) aswell as Hawthorne and Isaacs (2018).

One can construct versions of the designer hypothesis \(D\) that aretailored to fulfil the likelihood inequality \(\eqref{likelihoods}\)bydefining the designer as a being with both the intentionand ability to create life-friendly conditions. However, one mayquestion whether such tailored versions of the designer hypothesishave sufficient independent motivation and plausibility to deserveserious consideration in the first place. To use Bayesian terms, onemay hesitate to ascribe them non-negligible prior probabilities\(P(D)\).

Motivating a non-negligible prior \(P(D)\) for design is especiallychallenging in the framework of the ur-probability solution to theproblem of old evidence because it constrains the background evidenceto facts that do not entail the existence of life. Collins argues thatif we focus only on a limited class of constants \(C\), the backgroundevidence that we can use to motivate the prior \(P(D)\) is allowed to“includ[e] the initial conditions of the universe, the laws ofphysics, and the values of all the other constants except C”.But appeals to the sacred texts of religions cannot be used tomotivate the ascription of a non-negligible ur-prior \(P(D)\) becausethey presuppose, and thus entail, the existence of life. Notably, aspointed out by Monton, “[i]n formulating an urprobability forthe existence of God, one cannot take into account Biblical accountsabout Jesus” (2006: 418). According to Monton (2006: 419),proponents of the argument from fine-tuning for design may, however,try to motivate a non-negligible ur-prior \(P(D)\) by resorting toarguments for the existence of God that are either a priori, e.g.,ontological argument, or appeal only to very general empirical factsthat do not entail that the conditions are right for life, e.g., thecosmological argument. According to Swinburne (2004: ch. 5), thehypothesis of traditional theism is a simple one and, as such,warrants the ascription of a non-negligible prior.

3.5 An Alternative Argument from Fine-Tuning for Design

The argument from fine-tuning for design as reviewed inSection 3.1 treats the fact that life requires fine-tuned conditions asbackground knowledge and assesses the evidential significance of theobservation that life-friendly conditions obtain against thatbackground. An alternative argument from fine-tuning for design,explored by John Roberts (2012) and independently investigated andendorsed by Roger White (2011) in a reply to Weisberg (2010), treatsour knowledge that the conditions are right for life as backgroundinformation and assesses the rational impact of physicists’insight that life requires fine-tuned conditions against thisbackground. An advantage of this alternative is that it fits betterwith our actual epistemic situation: that the conditions are right forlife is something we have known for a long time; our actual newevidence is that the laws of physics—as White (2011) andWeisberg (2012) put it—arestringent rather thanlax in the constraints that they impose on the constants andboundary conditions if there is to be life.

The central likelihood inequality around which White’s versionof the argument revolves is

where “\(D\)” is, again, the designer hypothesis,“\(S\)” is the proposition that the laws are stringent(i.e., that life requires delicate fine-tuning of the constants) and“\(O\)” is our background knowledge that life exists(White 2011: 678). (See Roberts [2012: 296] for an assumption thatplays an analogous role as \(\eqref{alternative}\).) The inequality\(\eqref{alternative}\) expresses the statement that stringent lawsconfirm the designer hypothesis, given our background knowledge thatlife exists. Does it plausibly hold for reasonable probabilityassignments? White argues that it does and supports this claim bygiving a rigorous derivation of \(\eqref{alternative}\) fromassumptions that he regards as plausible. Crucial among them is theinequality

which White motivates by arguing that “the fact that the lawsput stringent conditions on life does not by itself provide anyevidenceagainst design” (White 2011: 678). Putdifferently, according to White, absent information that life exists,information that the laws are stringent does at least not speakagainst the existence of a designer.

Weisberg (2012) criticizes \(\eqref{alternative1}\)—and takeshis criticism to undermine \(\eqref{alternative}\)—arguing thatit is implausible by the design theorist’s own standards. Thedesign theorist holds a combination of views according to which, onthe one hand, life is more probable if there is a designer than ifthere is no designer and life is less probable if the laws arestringent rather than lax. If one adds to this combination of viewsthe assumption that none of the possible life-friendly conditions hashigher probability than the others, both if there is a designer and ifthere is no designer, it dictates that—bracketing knowledge thatlife exists—stringent laws speakagainst the existenceof a designer, i.e., it dictates \(P(D\mid S) < P(D\mid \neg S)\),contrary to \(\eqref{alternative1}\). Absent any evidence that lifeexists, evidence that the laws are stringent speaks against theexistence of life in that stringent laws make life unexpected.

A possible response for the design theorist, anticipated by Weisberg(2012: 713), would be to support \(\eqref{alternative1}\) by arguingthat the designer would plausiblyfirst choose eitherstringent or lax laws, sidestepping her intention to enable theexistence of life at that stage or actively preferring stringent laws,and onlythen choosing life-friendly constants. A problemwith this response, similar to the difficulties discussed inSection 3.4, is that we have little experience with cosmic designers and,therefore, difficulties to predict the hypothesized designer’spreferences and likely actions.

4. Fine-Tuning and the Multiverse

According to the multiverse hypothesis, there are multiple universes,some of them radically different from our own. Many of those whobelieve that fine-tuning for life requires some theoretical responseregard it as the main alternative beside the designer hypothesis. Theidea that underlies it is that, if there is a sufficiently diversemultiverse in which the conditions differ between universes, it isonly to be expected that there is at least one where they are rightfor life. As the strong anthropic principle highlights (seeSection 3.2), the universe in which we, as observers, find ourselves must be onewhere the conditions are compatible with the existence of observers.This suggests that, on the assumption that there is a sufficientlydiverse multiverse, it is neither surprising that there is at leastone universe that is hospitable to life nor—since we could nothave found ourselves in a life-hostile universe—that we findourselves in a life-friendly one. Many physicists (e.g., Susskind[2005], Greene [2011], Tegmark [2014]) and philosophers (e.g., Leslie[1989], Smart [1989], Parfit [1998], Bradley [2009]) regard this lineof thought as suggesting the inference to a multiverse as a rationalresponse to the finding that the conditions are right for life in ouruniverse despite the required fine-tuning.

4.1 The Argument from Fine-Tuning for the Multiverse as An Inference to An Anthropic Explanation?

The argument from fine-tuning for the multiverse as just sketched issometimes characterized as an inference to the multiverse as the bestexplanation of fine-tuning for life—an explanation which, inview of its appeal to anthropic reasoning, is sometimes characterizedas “anthropic” (e.g., Leslie 1986, 1989: ch. 6; McMullin1993: 376f., sect. 7; Bostrom 2002). It is controversial, however,whether this characterization is adequate. A paradigmatic anthropic“explanation”, characterized as such by Carter in theseminal paper (1974) that introduces the anthropic principles, isastrophysicist Robert Dicke’s (1961) account of coincidencesbetween large numbers in cosmology. A prominent example of such acoincidence is that the relative strength of electromagnetism andgravity as acting on an electron/proton pair is of roughly the sameorder of magnitude (namely, \(10^{40}\)) as the age of the universe,measured in natural units of atomic physics. Impressed by this andother coincidences, Dirac (1938) stipulated that they might holduniversally and as a matter of physical principle. He conjectured thatthe strength of gravity may decrease as the age of the universeincreases, which would indeed make it possible for the coincidence tohold across all cosmic times.

Dicke (1961), criticizing Dirac, argues that standard cosmology withtime-independent gravity suffices to account for the coincidence,provided that we take into account the fact that our existence is tiedto the presence of mainline stars like then sun and of variouschemical elements produced in supernovae. As Dicke shows, thisrequirement dictates that we could only have found ourselves in thatcosmic period in which the coincidence holds. Accordingly, contrary toDirac, there is no reason to assume that gravity varies with time tomake the coincidence unsurprising. Carter (1974) and Leslie (1986,1989: ch. 6) describe Dicke’s account as an “anthropicexplanation” of the coincidence that impressed Dirac, and Lesliediscusses it continuously with the argument from fine-tuning for themultiverse. (Earman [1987: 309], however, disputes that Dicke’saccount is adequately characterized as an “explanation”.)But whereas Dicke’s account of the coincidence uses life’sexistence as background knowledge to show that standard cosmologysuffices to make the coincidence expectable, the standard argumentfrom fine-tuning for the multiverse, as reviewed in what follows,treats life’s existence as requiring a theoretical response(rather than as background knowledge) and advocates the multiversehypothesis as the best such response. Friederich (2019b; 2021: ch. 6)outlines how to set up the fine-tuning argument for the multiverse sothat it uses anthropic reasoning analogously to the Dicke/Carteraccounts of large number coincidences.

4.2 The Argument from Fine-Tuning for the Multiverse Using Probabilities

More often than as an inference to the best explanation the argumentfrom fine-tuning for the multiverse is formulated using probabilities,in analogy to the argument from fine-tuning for design (seeSection 3.1). In a simple version of the argument, reasonable probabilityassignments are compared for a single-universe hypothesis \(U\) (wherethe universe has uniform laws and constants) and a rival multiversehypothesis \(M\) according to which there are many universes withconditions that differ between universes. (For the purposes of thediscussion about fine-tuning for life, hypotheses according to whichthere is only a single universe with constants that vary acrossspace-time qualify as versions of the multiverse hypothesis. They seemto be disfavoured by the available evidence, however, see Uzan 2003for a review.)

As in Bradley 2009, we consider as the fine-tuning evidence theproposition \(R\) that there is (at least) one universe with the rightconstants for life. Using Bayesian conditioning and Bayes’theorem one obtains for the ratio of the posteriors

If the multiverse according to \(M\) is sufficiently vast and varied,life unavoidably appears somewhere in it, so the conditional prior\(P(R\mid M)\) must be \(1\) (or very close to \(1\)). If we assumethat, on the assumption that there is only a single universe, it isimprobable that it has the right conditions for life (seeSection 2.1 for discussion), the conditional prior \(P(R\mid U)\) must be muchsmaller than \(1\). This gives \(P(R\mid M) \gg P(R\mid U)\), whichentails \(P(R\mid M)/P(R\mid U)\gg1\), which in turn entails a ratioof posteriors that is much larger than the ratio of the priors:\(\frac{P(M\mid R)}{P(U\mid R)}\gg\frac{P(M)}{P(U)}\). Unless we haveprior reasons to dramatically prefer a single universe over themultiverse, i.e., unless \(P(U)\gg P(M)\), the ratio of the posteriors\(\frac{P^+(M)}{P^+(U)}\) will be larger than \(1\).

Just as the argument from fine-tuning for design, the argument fromfine-tuning for the multiverse must come to terms with the problemthat the existence of life is old evidence for us. If one appliesHowson’s ur-probability solution to it, one must consistentlyinterpret all the probabilities in equation \(\eqref{simplemult}\) asassigned from the perspective of a counterfactual epistemic agent whois unaware of her/his own existence. At least prima facie, it isunclear what background knowledge can be assumed for an agent in thatcurious condition (seeSection 3.3 for considerations). Juhl (2007) speculates that motivating anon-negligible prior \(P(M)\) is impossible without implicitly relyingon evidence which entails that the conditions are right for life. Ifthis is correct, it means that running the fine-tuning argument forthe multiverse as in equation \(\eqref{simplemult}\) based on anempirically well motivated non-negligible prior \(P(M)\) wouldinevitably involve fallacious double-counting(“double-dipping”, as Juhl calls it (2007: 554)) of thefine-tuning evidence \(R\).

4.3 The Inverse Gambler’s Fallacy Charge

The inverse gambler’s fallacy, identified by Ian Hacking (1987),consists in inferring from an event with a remarkable outcome thatthere have likely been many more events of the same type in the past,most with less remarkable outcomes. For example, the inversegambler’s fallacy is committed by someone who enters a casinoand, upon witnessing a remarkable outcome at the nearesttable—say, a five-fold six in a quintuple dietoss—concludes that the toss is most likely part of a largesequence of tosses. Critics of the argument from fine-tuning for themultiverse accuse it of committing the inverse gambler’sfallacy. According to them, the argument commits this fallacy by, asWhite puts it,

supposing that the existence of many other universes makes it morelikely thatthis one—the only one that we haveobserved—will be life-permitting. (White 2000: 263)

Versions of this criticism are endorsed by Draper et al. (2007) andLandsman (2016). Hacking (1987) regards only those versions of theargument from fine-tuning for the multiverse as guilty of the inversegambler’s fallacy that infer the existence of multiple universesin a temporal sequence.

Adherents of the inverse gambler’s fallacy charge against theargument from fine-tuning for the multiverse object against focusingon the impact of the proposition \(R\)—that the conditions areright for life insome universe. According to them, we shouldinstead consider the impact of the more specific proposition \(H\):that the conditions are right for lifehere, inthisuniverse. If we replace \(R\) by \(H\), they argue, it becomes clearthat the argument breaks down because the existence of other universedoes not raise the probability that this universe here islife-friendly.

Many philosophers defend the argument from fine-tuning for themultiverse against this objection (McGrath 1988; Leslie 1988; Bostrom2002; Manson & Thrush 2003; Juhl 2005; Bradley 2009; Epstein2017). In an early response to Hacking, McGrath (1988) argues that theanalogy between the argument from fine-tuning for the multiverse and aperson who randomly enters a casino and witnesses a remarkable outcomeis misleading: while the person entering a casino could have found anyarbitrary outcome, we could not have found ourselves in a universewith conditions that are not right for life. The appropriate analogyto consider, according to McGrath, involves someone who is allowed toenter the casino only if and when some specific remarkable outcomeoccurs and who, upon being called in and finding that this outcome hasoccurred, infers the existence of other trials in the past. Inthat scenario, the inference to multiple trials (in the past)is indeed rational, and so, according to McGrath, is the inferencefrom fine-tuned conditions to multiple universes.

The adequacy of McGrath’s casino analogy is contested as well.Whereas in McGrath’s analogy the epistemic agent waits outsidethe casino until the remarkable outcome occurs and she/he is calledin, “it is not as though we were disembodied spirits waiting fora big bang to produce some universe which could accommodate us”,as White puts it (2000: 268). Epstein (2017: 653) retorts that“it is also not as though we were disembodied spirits keenlyobserving [the universe] ɑ—our designated potentialhome—and hoping that it, in particular, would be able toaccommodate us.” Epstein’s diagnosis is that the inversegambler’s fallacy charge rests on the Requirement of TotalEvidence in Bayesianism, which, according to him, is to be rejected incases like these. Draper (2020) as well as Barrett and Sober (2020)defend the Requirement of Total Evidence and, in doing so, attackEpstein’s criticism of the inverse gambler’s fallacycharge. Bradley (2009) offers further casino analogies beyond thoseconsidered by Hacking, McGrath and White which, according to him,speak in favour of rejecting the inverse gambler’s fallacycharge, but White’s diagnosis continues to find support, e.g.,by Landsman (2016). Friederich (2019a; 2021: ch. 4) suggests that thequestion of whether we can rationally infer a multiverse fromfine-tuning for life is so different from questions encountered inmore familiar contexts such as the casino scenarios that whether ornot the inference from fine-tuning to a multiverse commits the inversegambler’s fallacy may not have a determinate answer by thestandards of accepted rationality criteria.

4.4 Independently Motivating and Testing the Multiverse Hypothesis?

As just outlined, it is controversial whether it is rational to inferthe existence of multiple universes from our universe’sfine-tuning for life. However, if we had strongindependentevidence for other universes with life-hostile conditions, attempts toaccount for why our own universe is life-friendly would most likelyseem futile. Thus independent evidence for some multiverse scenariocould have a strong impact on what we regard as a rational response tofine-tuning for life. Proponents of the argument from fine-tuning forthe multiverse could moreover welcome such evidence as potentiallyhelping to motivate a non-negligible prior \(P(M)\) for themultiverse.

Many physicists nowadays believe that a specific version of themultiverse hypothesis is indeed suggested by contemporary developmentsin fundamental physics, notably by the combination of inflationarycosmology and string theory, both of which have been introduced inSection 2.3. According to many advocates of inflationary cosmology, the process ofinflation results in causally isolated space-time regions, so-called“island universes”. This process is in general“eternal” in that the formation of island universes neverends. As a result, it leads to the production of a vast (and,according to most models, infinite) “multiverse” of islanduniverses (Guth 2000).

As remarked inSection 2.3, string theory has an enormous number of lowest energy states (vacua),which would manifest themselves at the level of observations andexperiments in terms of different higher level physical laws andvalues of the constants. When combined with the idea of islanduniverses as suggested by inflationary cosmology one obtains acosmological picture in which there are infinitely many islanduniverses where all the different string theoryvacua—corresponding to different higher-level physical laws andconstants in these laws—are actually realized in the differentisland universes. This so-calledlandscape multiversequalifies as a concrete multiverse scenario in the sense of theargument from fine-tuning for the multiverse. A necessary condition isof course that the collection of island universes that are part of thelandscape multiverse includes, as is widely believed to be the case,at least one universe with the same effective (higher-level) laws andconstants as our own.

Unfortunately, concrete multiverse scenarios such as the landscapemultiverse are extremely difficult to test, precisely because theyentail that different universes exhibit very different conditions. Thebroad consensus in the literature on multiverse cosmology is that, inorder for a multiverse scenario to qualify as empirically confirmed,it must entail that those conditions that we find in our own universearetypical among those found by observers across themultiverse. Widely used formulations of typicality areVilenkin’sprinciple of mediocrity (Vilenkin 1995) andBostrom’sself-sampling assumption (Bostrom 2002).Typicality principles can be regarded as refinements of the anthropicprinciples (Bostrom 2002) in the form ofindifference principlesof self-locating belief (Elga 2004): inasmuch as we are ignorantabout who and where among observers we are, they recommend to reasonas if we were equally likely to be any of the observers who wemight possibly be, given our empirical evidence.

Typicality principles have the benefit of making multiverse theoriesat least in principle testable (Aguirre 2007; Barnes 2017). They arecontroversial, however, because it is contested whether typicality isalways a reasonable assumption (Hartle & Srednicki 2007; Smolin2007) and because it is difficult to specify with respect to whichreference class of observers typicality should be assumed. Inpractice, observer proxies are chosen such as share of baryon matterclustered in large galaxies (Martel et al. 1998) or entropy gradient(Bousso et al. 2007). These difficulties are exacerbated incosmological scenarios such as the landscape multiverse in whichreference classes of observers that one might reasonably choose areall infinite. The problem of regularizing those infinities correspondsto the so-calledmeasure problem of cosmology, according tosome cosmologists “the greatest crisis in physics today”(Tegmark 2014: 314). (See Schellekens [2013: sect. VI.B] for anintroduction to the measure problem aimed at physicists, Smeenk [2014]for a philosopher’s sceptical assessment of its solvability, andDorr & Arntzenius [2017] for a more optimistic perspective.)Friederich (2021: ch. 8) argues that the freedom to choose, forexample, an observer proxy and a cosmic measure makes typicality-basedpredictions from multiverse theories untrustworthy and susceptible toconfirmation bias.

The persisting difficulties with testing multiverse theories are aprime reason why the multiverse idea itself continues to be viewedvery critically by many leading physicists (e.g., Ellis 2011).

5. Fine-Tuning and Naturalness

According to many contemporary physicists, the most deeply problematicinstances of fine-tuning do not concern fine-tuning for life butviolations ofnaturalness—a principle of theory choicein particle physics and cosmology that can be characterized as ano fine-tuning criterion.

5.1 IntroducingNaturalness

The idea that underliesnaturalness is that the phenomenadescribed by some physical theory should not depend sensitively onspecific details of a more fundamental (currently unknown) theory towhich it is an effective low-energy approximation. In what follows,the motivation, significance, and implementation of this idea in theframework of quantum field theory are explained. For a more detailedintroduction aimed at physicists see Giudice (2008), for one aimed atphilosophers of physics see Williams (2015).

Modern physics regards our currently best theories of particle physicscollected in the Standard Model aseffective field theories.Effective field theories are low-energy effective approximations tohypothesized more fundamental physical theories whose details arecurrently unknown. An effective field theory has an in-built limit toits range of applicability, determined by some energy scale\(\Lambda\). When applied to phenomena associated with energies higherthan \(\Lambda\) the effective field theory will not deliver correctresults. At this point, the more fundamental theory must be consideredto which the effective field theory supposedly is a low-energyapproximation. For the theories collected in the Standard Model, it isknown that they cannot possibly be empirically adequate beyondenergies around the Planck scale \(\Lambda_{\textrm{Planck}}\approx10^{19} \,\textrm{GeV}\), where—presently unknown—quantumgravitational effects become relevant. However, the Standard Model maywell be empirically inadequate already at energy scales significantlybelow the Planck scale. For example, if there is some presentlyunknown particle with mass \(M\) smaller than the Planck scale\(\Lambda_{\textrm{Planck}}\) but beyond the range of currentaccelerator technology which interacts with particles described by theStandard Model, the cut-off scale \(\Lambda\) where the Standard Modelbecomes inadequate may well be \(M\) rather than\(\Lambda_{\textrm{Planck}}\).

In an effective field theory, any physical quantity \(g_{\Fphys}\) canbe represented as the sum of a so-called bare quantity \(g_0\) and acontribution \(\Delta g\) from vacuum fluctuations corresponding toenergies up to the cut-off \(\Lambda\):

The bare quantity \(g_0\) can be regarded as a black box that sums upeffects associated with energies beyond the cut-off scale \(\Lambda\)where unknown effects must be taken into account. Viewing a theory asan effective field theory means viewing it as a self-containeddescription of phenomena up to the cut-off scale \(\Lambda\). Thisperspective suggests that one may only consider an effective theory asnatural if the physical quantity \(g_{\Fphys}\) can be of itsactual order of magnitude without any need for a delicate cancellationbetween \(g_0\) and \(\Delta g\) to many orders of magnitude. Sincethe bare quantity \(g_0\) sums up information about physics beyond thecut-off scale \(\Lambda\), such a delicate cancellation between\(g_0\) and \(\Delta g\) would mean that the order of magnitude of thephysical quantity \(g_{\Fphys}\) would be different if phenomenaassociated with energies beyond the cut-off scale \(\Lambda\) wereslightly different.

One can characterize violations of naturalness as instances offine-tuning in that, where naturalness is violated,low-energy phenomena depend sensitively on the details of some unknownfundamental theory concerning phenomena at very high energies.Physicists have developed ways of quantifying fine-tuning in thissense (Barbieri & Guidice 1988), critically discussed by Grinbaum(2012). It is controversial whether, to the degree that violations ofnaturalness can be seen as instances of fine-tuning, they should beregarded as problematic. Wetterich (1984) suggests that anyfine-tuning of bare parameters is unproblematic because thoseparameters depend on the chosen regularization scheme and have noindependent physical meaning. As highlighted by Rosaler and Harlander(2019), Wetterich’s perspective depends on an understanding ofquantum field theories as defined by entire trajectories\(g^i(\Lambda)\) in parameter space.

An alternative criterion of naturalness—sometimes dubbedabsolute naturalness (see Wells [2015] for an empiricalmotivation)—is that a theory is natural if and only if it can beformulated using dimensionless numbers that are all of order \(1\).More permissive is ’t Hooft’stechnicalnaturalness criterion (’t Hooft 1980), according to which atheory is natural if it can be formulated in terms of numbers that areeither of order \(1\) or very small but such that, if they wereexactly zero, the theory would have an additional symmetry. Themotivation for this prima facie arbitrary criterion is that itelegantly reproduces verdicts based on the above formulation ofnaturalness according to which low-energy phenomena should not dependsensitively on the details of some more fundamental theory withrespect to high energies.

5.2 Violations of Naturalness: Examples

A prime example of a violation of naturalness occurs in quantum fieldtheories with a spin \(0\) scalar particle such as the Higgs particle.In this case, the dependence of the squared physical mass on thecut-off \(\Lambda\) is quadratic:

The physical mass of the Higgs particle is empirically known to be\(m_{\FH,\Fphys}\approx125\,\textrm{GeV}\). The dominant contributionto \(\Delta m^2\), specified as \(h_t\Lambda^2\) in equation\(\eqref{Higgs}\), is due to the interaction between the Higgsparticle and the heaviest fermion, the top quark, where \(h_t\) issome parameter that measures the strength of that interaction. Giventhe empirically known properties of the top quark, the factor\(\frac{h_t}{16\pi^2}\) is of order \(10^{-2}\). Due to its quadraticdependence on the cut-off scale \(\Lambda\) the term\(\frac{h_t}{16\pi^2}\Lambda^2\) is very large if the cut-off scale islarge. If the Standard Model is valid up to the Planck scale\(\Lambda_{\textrm{Planck}}\approx10^{19}\,\textrm{GeV}\), the squaredbare mass \(m_{\FH,0}^2\) and the effect of the vacuum fluctuationswould have to cancel each other out to about 34 orders of magnitude inorder to result in a physical Higgs mass of \(125\,\textrm{GeV}\).There is no known physical reason why the effects collected in thebare mass \(m_{\FH}\) should be in such a delicately balance with theeffects from the vacuum fluctuations collected in \(\Delta m^2\). Thefact that two fundamental scales—the Planck scale and the Higgsmass—are so widely separated from each other is referred to asthehierarchy problem. As a consequence of this problem, theviolation of naturalness due to the Higgs mass is so severe.

Various solutions to the naturalness problem for the Higgs mass havebeen proposed in the form of theoretical alternatives to the StandardModel. In supersymmetry (see Martin [1998] for an introduction),contributions to \(\Delta m_{\FH}^{2}\) from supersymmetric partnerparticles can compensate the contribution from heavy fermions such asthe top quark and thereby eliminate the fine-tuning problem. However,supersymmetric theories with this feature appear to be disfavoured bymore recent experimental results, notably from the Large HadronCollider (Draper et al. 2012). Other suggested solutions to thenaturalness problem for the Higgs particle include so-calledTechnicolour models (Hill & Simmons 2003), in which theHiggs particle is replaced by additional fermionic particles, modelswith large extra dimensions, where the hierarchy between the Higgsmass and the Planck scale is drastically diminished (Arkani-Hamed etal. 1998), and models with so-calledwarped extra dimensions(Randall & Sundrum 1999).

An even more severe violation of naturalness is created by thecosmological constant \(\rho_V\), which specifies the overall vacuumenergy density. Here the contribution due to vacuum fluctuations isproportional to the fourth power of the cut-off scale \(\Lambda\):

The physical value \(\rho_V\) of the cosmological constant isempirically found to be of order \(\rho_V\sim10^{-3}\,\textrm{eV}\).The constant \(c\), which depends on parameters that characterize thetop quark and the Higgs particle, is empirically known to be roughlyof order \(1\). If we take the cut-off to be of the order of thePlanck scale \(\Lambda\sim10^{19}\,\textrm{GeV}\), the bare term\(\rho_0\), must cancel the contribution \(c\Lambda^4\) to more than120 orders of magnitude. Even if we assume a cut-off as low as\(\Lambda\sim1\,\textrm{TeV}\), i.e., already within reach of currentaccelerator technology, we find that a cancellation between \(\rho_0\)and \(c\Lambda^4\) to about 50 digits remains necessary. Contrary tothe case of the Higgs mass, there are few ideas of how future physicaltheories might be able to avoid this problem.

5.3 Violations of Naturalness and Fine-Tuning for Life

As explained inSection 5.1, violations of naturalness can be seen as instances of fine-tuning,but not in the sense of fine-tuning for life. A connection betweennaturalness and fine-tuning for life can be constructed, however,along the following lines:

One can interpret equations \(\eqref{Higgs}\) and\(\eqref{cosmo_constant}\) as suggesting that the actual physicalvalues of the Higgs mass and the cosmological constant are muchsmaller than the values that one would expect for them in theframework of the Standard Model. Notably, if the Higgs mass were oforder of the cut-off \(\Lambda\), e.g., the Planck scale, and if thecosmological constant were of order \(\Lambda^4\), the bare parameterswould not need to be fixed to many digits in order for the physicalparameters to have their respective orders of magnitude, which meansthat the physical values would be natural. Thus, assuming naturalnessand the validity of our currently best physical theories up to thePlanck scale, one would expect values for the Higgs mass and thecosmological constants of the same order of magnitude as their vacuumcontributions, i.e., values much larger than the actual ones.

With respect to the problem of specifying probability distributionsover possible values of physical parameters discussed inSection 2.1 naturalness may be taken to suggest that all reasonable suchdistributions have most of their probabilistic weight close to thenatural values. As explained, for the Higgs mass and thecosmological constant the natural values are much larger than theobserved ones. Advocates of the view that fine-tuning for liferequires a response because life-friendly constants are improbabletherefore put particular emphasis on those instances of fine-tuningfor life that are associated with violations of naturalness, notablythe cosmological constant (e.g., Susskind 2005: ch. 2; Donoghue 2007;Collins 2009: sect. 2.3.3; Tegmark 2014: 140f.).

Bibliography

• Adams, Fred C., 2008, “Stars in other universes: stellarstructure with different fundamental constants”,Journal ofCosmology and Astroparticle Physics, 08: 10.doi:10.1088/1475-7516/2008/08/010
• Adams, Fred C., 2019, “The degree of fine-tuning in ouruniverse – and others”,Physics Reports, 807:1–111. doi:10.1016/j.physrep.2019.02.001
• Aguirre, Anthony, 2007, “Making predictions in a multiverse:conundrums, dangers, coincidences”, in Carr 2007: 367–386.doi:10.1017/CBO9781107050990.023
• Arkani-Hamed, Nima, Savas Dimopoulos, and Gia Dvali, 1998,“The Hierarchy problem and new dimensions at amillimeter”,Physics Letters B, 429(3–4):263–272. doi:10.1016/S0370-2693(98)00466-3
• Barbieri, Riccardo and Gian F. Giudice, 1988, “Upper boundson supersymmetric particle masses”,Nuclear Physics B,306(1): 63–76. doi:10.1016/0550-3213(88)90171-X
• Barnes, Luke A., 2012, “The fine-tuning of the universe forintelligent life”,Publications of the Astronomical Societyof Australia, 29(4): 529–564. doi:10.1071/AS12015
• –––, 2017, “Testing the multiverse: Bayes,fine-tuning and typicality”, in Chamcham et al. 2017:447–466. doi:10.1017/9781316535783.023
• –––, 2020, “A reasonable little question:a formulation of the fine-tuning argument”,Ergo, 6:1220–1257. doi:10.3998/ergo.12405314.0006.042
• Barr, S. M. and A. Khan, 2007, “Anthropic tuning of the weakscale and of \(m_u/m_d\) in two-Higgs-doublet models”,Physical Review D, 76(4): 045002doi:10.1103/PhysRevD.76.045002
• Barrett, Martin and Sober, Elliott, 2020, “The Requirementof Total Evidence: a reply to Epstein’s critique”,Philosophy of Science, 87(1): 191–203.doi:10.1086/706086
• Barrow, John D. and Frank J. Tipler, 1986,The AnthropicCosmological Principle, Oxford: Oxford University Press.
• Behe, Michael J., 1996,Darwin’s Black Box, NewYork: The Free Press.
• Bostrom, Nick, 2002,Anthropic Bias: Observation SelectionEffects in Science and Philosophy, New York: Routledge.
• Bradley, Darren J., 2009, “Multiple universes andobservation selection effects”,American PhilosophicalQuarterly, 46: 61–72.
• Carlson, Erik and Erik J. Olsson, 1998, “Is our existence inneed of further explanation?”,Inquiry, 41(3):255–275. doi:10.1080/002017498321760
• Carr, Bernard J. (ed.), 2007,Universe or Multiverse?,Cambridge: Cambridge University Press.doi:10.1017/CBO9781107050990
• Carr, Bernard J. and Martin J. Rees, 1979, “The anthropicprinciple and the structure of the physical world”,Nature, 278: 605–612. doi:10.1038/278605a0
• Carter, B., 1974, “Large number coincidences and theanthropic principle in cosmology”, in M. S. Longair (ed.),Confrontation of Cosmological Theory with Observational Data,Dordrecht: Reidel, pp. 291–298.
• –––, 1983, “The anthropic principle andits implications for biological evolution”,PhilosophicalTransactions of the Royal Society A, 310(1512): 347–363.doi:10.1098/rsta.1983.0096
• Chamcham, Khalil, Joseph Silk, John D. Barrow, and Simon Saunders(eds.), 2017,The Philosophy of Cosmology, Cambridge:Cambridge University Press. doi:10.1017/9781316535783
• Collins, R., 2009, “The teleological argument: anexploration of the fine-tuning of the cosmos”, in W. L. Craigand J.P. Moreland (eds.),The Blackwell Companion to NaturalTheology, Oxford: Blackwell, pp. 202–281.
• Colyvan M., J. L. Garfield, and G. Priest, 2005, “Problemswith the argument from fine-tuning”,Synthese, 145(39):325–338. doi:10.1007/s11229-005-6195-0
• Craig, William Lane, 2003, “Design and the anthropicfine-tuning of the universe”, in Manson 2003:155–177.
• Davies, Paul C.W., 2006,The Goldilocks Enigma: Why is theUniverse Just Right for Life?, London: Allen Lane.
• Dawid, Richard, 2013,String Theory and the ScientificMethod, Cambridge: Cambridge University Press.doi:10.1017/CBO9781139342513
• Dembski, William A., 1998,The Design Inference: EliminatingChance through Small Probabilities, Cambridge: CambridgeUniversity Press. doi:10.1017/CBO9780511570643
• Dicke, R. H., 1961, “ Dirac’s cosmology andMach’s principle”,Nature, 192: 440–441.doi:10.1038/192440a0
• Dirac, P.A.M., 1938, “A new basis for cosmology”,Proceedings of the Royal Society A, 165: 199–208.doi:10.1098/rspa.1938.0053
• Donoghue, John F., 2007, “The fine-tuning problems ofparticle physics and anthropic mechanisms”, in Carr 2007:231–246. doi:10.1017/CBO9781107050990.017
• Dorr, Cian and Frank Arntzenius, 2017, “Self-locating priorsand cosmological measures”, in Chamcham et al. 2017:396–428. doi:10.1017/9781316535783.021
• Draper, Kai, Paul Draper, and Joel Pust, 2007,“Probabilistic arguments for multiple universes”,Pacific Philosophical Quarterly, 88(3): 288–307.doi:10.1111/j.1468-0114.2007.00293.x
• Draper, Paul, 2020, “In defense of the requirement of totalevidence”,Philosophy of Science, 87(1): 179–190.doi:10.1086/706084
• Draper, Patrick, Patrick Meade, Matthew Reece, and David Shih,2012, “Implications of a 125 GeV Higgs boson for the MSSM andlow-scale supersymmetry breaking”,Physical Review D,85(9): 095007. doi:10.1103/PhysRevD.85.095007
• Earman, John, 1987, “The SAP also rises: a criticalexamination of the anthropic principle”,AmericanPhilosophical Quarterly, 24(4): 307–317.
• Earman, John and Jesus Mosterín, 1999, “A criticallook at inflationary cosmology”,Philosophy of Science,66(1): 1–49. doi:10.1086/392675
• Einstein, Albert, 1949, “Autobiographical notes”, inP.A. Schilpp (ed.),Albert Einstein: Philosopher-Scientist,Peru, IL: Open Court Press.
• Elga, Adam, 2004, “Defeating Dr. Evil with self-locatingbelief”,Philosophy and Phenomenological Research,69(2): 383–396. doi:10.1111/j.1933-1592.2004.tb00400.x
• Ellis, George F.R., 2011, “Does the multiverse reallyexist?”,Scientific American, 305: 38–43.doi:10.1038/scientificamerican0811-38
• Epstein, Peter F., 2017, “The fine-tuning argument and therequirement of total evidence”,Philosophy of Science,84(4): 639–658. doi:10.1086/693465
• Friederich, Simon, 2019a, “Reconsidering the inversegamblers fallacy charge against the fine-tuning argument for themultiverse”,Journal for General Philosophy of Science,50: 155–178. doi:10.1007/s10838-018-9422-3
• –––, 2019b, “A new fine-tuning argumentfor the multiverse”,Foundations of Physics, 49:1011–1021. doi:10.1007/s10701-019-00246-2
• –––, 2021,Multiverse Theories: APhilosophical Perspective, Cambridge: Cambridge UniversityPress.
• Giudice, Gian Francesco, 2008, “Naturally speaking: thenaturalness criterion and physics at the LHC”, in Gordon L. Kaneand Aaron Pierce (eds.),Perspectives on LHC physics,Singapore: World Scientific, pp. 155–178.doi:10.1142/9789812779762_0010
• Glymour, Clark N., 1980,Theory and Evidence, Princeton,NJ: Princeton University Press.
• Gould, Stephen Jay, 1983, “Mind and supermind”,Natural History, 92(5): 34–38.
• Greene, Brian, 2011,The Hidden Reality: Parallel Universesand the Deep Laws of the Cosmos, New York: Vintage.
• Grinbaum, Alexei, 2012, “Which fine-tuning arguments arefine?”,Foundations of Physics, 42(5): 615–631.doi:10.1007/s10701-012-9629-9
• Grohs, Evan, Howe, Alex R., and Adams, Fred C., 2018,“Universes without the weak force: Astrophysical processes withstable neutrons”,Physical Review D, 97: 043003.doi:10.1103/PhysRevD.97.043003
• Guth, Alan H., 1981, “Inflationary universe: A possiblesolution to the horizon and flatness problems”,PhysicalReview D, 23(2): 347. doi:10.1103/PhysRevD.23.347
• –––, 2000, “Inflation and eternalinflation”,Physics Reports, 333: 555–574.doi:10.1016/S0370-1573(00)00037-5
• Hacking, Ian, 1987 “The inverse gambler’s fallacy: theargument from design. The anthropic principle applied to WheelerUniverses”,Mind, 96(383): 331–340.doi:10.1093/mind/XCVI.383.331
• Hall, Lawrence J., David Pinner, and Joshua T. Ruderman, 2014,“The weak scale from BBN”,Journal of High EnergyPhysics, 2014(12): 134. doi:10.1007/JHEP12(2014)134
• Harnik, Roni, Graham D. Kribs, and Gilad Perez, 2006, “Auniverse without weak interactions”,Physical Review D,74(3): 035006. doi:10.1103/PhysRevD.74.035006
• Hartle, James B. and Mark Srednicki, 2007, “Are wetypical?”,Physical Review D, 75(12): 123523.doi:10.1103/PhysRevD.75.123523
• Hawthorne, John and Yoaav Isaacs, 2018, “Fine-tuningfine-tuning”, in Matthew A. Benton, John Hawthorne, and DaniRabinowitz (eds.),Knowledge, Belief, and God: New Insights inReligious Epistemology, Oxford: Oxford University Press, pp.136–168. doi:10.1093/oso/9780198798705.001.0001
• Hill, Christopher T. and Elizabeth H. Simmons, 2003, “Strongdynamics and electroweak symmetry breaking”,PhysicsReports, 381(4–6): 235–-402.doi:10.1016/S0370-1573(03)00140-6
• Hogan, Craig J., 2000, “Why the universe is just so”,Reviews of Modern Physics, 72: 1149–1161.doi:10.1103/RevModPhys.72.1149
• –––, 2007, “Quarks, electrons, and atomsin closely related universes”, in Carr 2007: 221–230.doi:10.1017/CBO9781107050990.016
• Holder, Rodney D., 2002, “Fine-tuning, multiple universesand theism”,Noûs, 36(2): 295–312.doi:10.1111/1468-0068.00372
• Howson, Colin, 1991, “The ‘old evidence’problem”,British Journal for the Philosophy ofScience, 42(4): 547–555. doi:10.1093/bjps/42.4.547
• Hoyle, Fred, D.N.F. Dunbar, W.A. Wenzel, and W. Whaling, 1953,“A state in C¹² predicted from astrophysicalevidence”,Physical Review, 92: 1095.
• van Inwagen, Peter, 1993,Metaphysics, Colorado: WestviewPress.
• Juhl, Cory, 2005, “Fine-tuning, many worlds, and the‘inverse gambler’s fallacy’”,Noûs, 39(2): 337–347.doi:10.1111/j.0029-4624.2005.00504.x
• –––, 2006, “Fine-tuning is notsurprising”,Analysis, 66(4): 269–275.doi:10.1111/j.1467-8284.2006.00628.x
• –––, 2007, “Fine-tuning and oldevidence”,Noûs, 41(3): 550–558.doi:10.1111/j.1468-0068.2007.00661.x
• Keynes, John, M., 1921,A Treatise on Probability,London: Macmillan.
• Koperski, Jeffrey, 2005, “Should we care aboutfine-tuning?”,British Journal for the Philosophy ofScience, 56(2): 303–319. doi:10.1093/bjps/axi118
• Kotzen, Matthew, 2012, “Selection biases in likelihoodarguments”,British Journal for the Philosophy ofScience, 63(4): 825–839. doi:10.1093/bjps/axr044
• Landsman, Klaas, 2016, “The fine-tuning argument: exploringthe improbability of our own existence”, in K. Landsman and E.van Wolde (eds.),The Challenge of Chance, Heidelberg:Springer, pp. 111–129. doi:10.1007/978-3-319-26300-7_6
• Leslie, John, 1986, “Anthropic explanations incosmology”,PSA: Proceedings of the Biennial Meeting of thePhilosophy of Science Association, pp. 87–95.
• –––, 1988, “No inverse gambler’sfallacy in cosmology”,Mind, 97(386): 269–272.doi:10.1093/mind/XCVII.386.269
• –––, 1989,Universes, London:Routledge.
• Lewis, Geraint J. and Luke A. Barnes, 2016,A FortunateUniverse: Life in a Finely Tuned Cosmos, Cambridge: CambridgeUniversity Press. doi:10.1017/CBO9781316661413
• MacDonald, J. and D.J. Mullan, 2009, “Big bangnucleosynthesis: The strong nuclear force meets the weak anthropicprinciple”,Physical Review D, 80(4): 043507.doi:10.1103/PhysRevD.80.043507
• Manson, Neil A. (ed.), 2003,God and Design: The TeleologicalArgument and Modern Science, London: Routledge.
• –––, 2009, “The fine-tuningargument”,Philosophy Compass, 4(1): 271–286.doi:10.1111/j.1747-9991.2008.00188.x
• Manson, Neil A. and Michael J. Thrush, 2003, “Fine-tuning,multiple universes, and the ‘this universe’objection”,Pacific Philosophical Quarterly, 84(1):67–83. doi:10.1111/1468-0114.00163
• Manson, Neil A., 2020, “How not to be generous tofine-tuning sceptics”,Religious Studies, 56(3):303–317. doi:10.1017/S0034412518000586
• Martin, Stephen P., 1998, “A supersymmetry primer”, inGordon L. Kane (ed.), Perspectives on Supersymmetry, Singapore: WorldScientific, pp. 1–98. doi:10.1142/9789812839657_0001
• McCoy, C.D., 2015, “Does inflation solve the hot big bangmodel’s fine-tuning problems?”,Studies in History andPhilosophy of Modern Physics, 51: 23–36.doi:10.1016/j.shpsb.2015.06.002
• McGrath, P.J., 1988, “The inverse gambler’s fallacyand cosmology—a reply to Hacking”,Mind, 97(386):331–340. doi:10.1093/mind/XCVII.386.265
• McGrew, Timothy, Lydia McGrew, and Eric Vestrup, 2001,“Probabilities and the fine-tuning argument: a scepticalview”,Mind, 110(449): 1027–1038.doi:10.1093/mind/110.440.1027
• McMullin, Ernan, 1993, “Indifference principle and anthropicprinciple in cosmology”,Studies in History and Philosophyof Science, 24(3): 359–389.doi:10.1016/0039-3681(93)90034-H
• Miller, Kenneth R., 1999,Finding Darwin’s God: AScientist’s Search for Common Ground between God andEvolution, New York: Cliff Street Books.
• Monton, Bradley, 2006, “God, fine-tuning, and the problem ofold evidence”,British Journal for the Philosophy ofScience, 57(2): 405–424. doi:10.1093/bjps/axl008
• Narveson, Jan, 2003, “God by design?”, in Manson 2003:88–105.
• Oberhummer, H., A. Csótó, and H. Schlattl, 2000,“Stellar production rates of carbon and its abundance in theuniverse”,Science, 289(5476): 88–90.doi:10.1126/science.289.5476.88
• Paley, William, 1802,Natural Theology: or, Evidences of theExistence and and Attributes of the Deity, Collected from theAppearances of Nature, London: Rivington.
• Parfit, Derek, 1998, “Why anything? Why this?”,London Review of Books, January 22: 24–27.
• Penrose, Roger, 2004,The Road to Reality: A Complete Guide tothe Laws of the Universe, London: Vintage.
• Planck Collaboration, 2014, “Planck 2013 results. I.Overview of products and scientific results”,Astronomy andAstrophysics, 571: A1. doi:10.1051/0004-6361/201321529
• Randall, Lisa and Raman Sundrum, 1999, “Large mass hierarchyfrom a small extra dimension”,Physical Review Letters,83(17): 3370–-3373. doi:10.1103/PhysRevLett.83.3370
• Rees, Martin, 2000,Just Six Numbers: the Deep Forces thatShape the Universe, New York: Basic Books.
• Rickles, Dean, 2014,A Brief History of String Theory,Berlin, Heidelberg: Springer.
• Roberts, John T., 2012, “Fine-tuning and the infraredbull’s eye”,Philosophical Studies, 160(2):287–303. doi:10.1007/s11098-011-9719-0
• Rosaler, Joshua and Harlander, Robert, 2019, “Naturalness,Wilsonian renormalization and ‘fundamentalparameters’” in quantum field theory,Studies inHistory and Philosophy of Modern Physics, 66: 118–134.doi:10.1016/j.shpsb.2018.12.003
• Rota, Michael, 2016,Taking Pascal’s Wager: Faith,Evidence and the Abundant Life, Downers Grove, IL: IntervarsityPress.
• Schellekens, A.N., 2013, “Life at the interface of particlephysics and string theory”,Reviews of Modern Physics,85(4): 1491. doi:10.1103/RevModPhys.85.1491
• Sloan, David, Rafael A. Batista, Michael T. Hicks and Roger Davies(eds.), 2020,Fine-tuning in the Physical Universe,Cambridge: Cambridge University Press.
• Smart, J.J.C., 1989,Our Place in the Universe: A MetaphysicalDiscussion, Oxford: Blackwell.
• Smeenk, Chris, 2014, “Predictability crisis in earlyuniverse cosmology”,Studies in History and Philosophy ofModern Physics, 46(1): 122–133.doi:10.1016/j.shpsb.2013.11.003
• Smolin, Lee, 2007, “Scientific alternatives to the anthropicprinciple”, in Carr 2007: 323–366.doi:10.1017/CBO9781107050990.022
• Sober, Elliott, 2003, “The design argument”, in Manson2003: 27–54.
• –––, 2009, “Absence of evidence andevidence of absence: evidential transitivity in connection withfossils, fishing, fine-tuning and firing squads”,Philosophical Studies, 143(1): 63–90.doi:10.1007/s11098-008-9315-0
• Steinhardt, Paul J. and Neil Turok, 2008,Endless Universe:Beyond the Big Bang, New York: Doubleday.
• Stenger, Victor J., 2011,The Fallacy of Fine-tuning: Why theUniverse Is Not Designed for Us, New York: Prometheus Books.
• Susskind, Leonard, 2005,The Cosmic Landscape: String Theoryand the Illusion of Intelligent Design, New York: Back BayBooks.
• Swinburne, Richard, 2003, “The argument to God fromfine-tuning reassessed”, in Manson 2003: 105–123.
• –––, 2004,The Existence of God, secondedition, Oxford: Clarendon.doi:10.1093/acprof:oso/9780199271672.001.0001
• Tegmark, Max, 2014,Our Mathematical Universe: My Quest forthe Ultimate Nature of Reality, New York: Knopf.
• Tegmark, Max and Martin J. Rees, 1998, “Why is the cosmicmicrowave background fluctuation level \(10^{-5}\)”,TheAstrophysical Journal, 499(2): 526–532.doi:10.1086/305673
• Tegmark, Max, Anthony Aguirre, Martin J. Rees, and Frank Wilczek,2006, “Dimensionless constants, cosmology, and other darkmatters”,Physical Review D, 73(2): 023505.doi:10.1103/PhysRevD.73.023505
• ’t Hooft, G., 1980, “Naturalness, chiral symmetry andspontaneous chiral symmetry breaking”, in G. ’t Hooft(ed.),Recent Developments in Gauge Theories, New York:Plenum Press, pp. 135–157.
• Uzan, Jeanne-Philippe, 2003, “The fundamental constants andtheir variation: observational and theoretical status”,Reviews of Modern Physics, 75(2): 403.doi:10.1103/RevModPhys.75.403
• –––, 2011, “Varying constants, gravitationand cosmology”,Living Reviews in Relativity, 14: 2.doi:10.12942/lrr-2011-2
• Venn, John, 1866,The Logic of Chance, New York:Chelsea.
• Vilenkin, Alexander, 1995, “Predictions from quantumcosmology”,Physical Review Letters, 74(6):846–849. doi:10.1103/PhysRevLett.74.846
• Weinberg, Steven, 1987, “Anthropic bound on the cosmologicalconstant”,Physical Review Letters, 59(22): 2607.doi:10.1103/PhysRevLett.59.2607
• Weisberg, Jonathan, 2005, “Firing squads and fine-tuning:Sober on the design argument”,British Journal for thePhilosophy of Science, 56(4): 809–821.doi:10.1093/bjps/axi139
• –––, 2010, “A note on design: what’sfine-tuning got to do with it?”,Analysis, 70(3):431–438. doi:10.1093/analys/anq028
• –––, 2012, “The argument from divineindifference”,Analysis, 72(4): 707–714.doi:10.1093/analys/ans113
• Wells, James D., 2015, “The utility of Naturalness, and howits application to Quantum Electrodynamics envisages the StandardModel and Higgs boson”,Studies in History and Philosophy ofModern Physics, 49: 102–108.doi:10.1016/j.shpsb.2015.01.002
• Wetterich, Christof, 1984, “Fine-tuning problem and therenormalization group”,Physical Review B, 140:215–222.
• White, Roger, 2000, “Fine-tuning and multipleuniverses”,Noûs, 34(2): 260–267.doi:10.1111/0029-4624.00210
• –––, 2011, “What’s fine-tuning gotto do with it: a reply to Weisberg”,Analysis, 71(4):676–679. doi:10.1093/analys/anr100
• Williams, Porter, 2015, “Naturalness, the autonomy ofscales, and the 125GeV Higgs”,Studies in History andPhilosophy of Modern Physics, 51: 82–96.doi:10.1016/j.shpsb.2015.05.003

Academic Tools

	How to cite this entry.
	Preview the PDF version of this entry at theFriends of the SEP Society.
	Look up topics and thinkers related to this entry at the Internet Philosophy Ontology Project (InPhO).
	Enhanced bibliography for this entryatPhilPapers, with links to its database.

Other Internet Resources

• Nick Bostrom’s webpage on the anthropic principles and related matters
• Martin Rees on “many universes” as a candidate response to fine-tuning for life
• Sabine Hossenfelder on why the case for cosmic fine-tuning is shaky
• Paul J. Steinhardt on why the multiverse idea is ready for retirement
• Martin Rees on why the multiverse idea deserves to be better known
• controversy between Susskind and Smolin over the anthropic principle
• Fred Adams on why the universe is not so fine-tuned after all
• Simon Friederich on why we may live in a multiverse but may never know

Related Entries

cosmology: and theology |creationism |epistemology, formal |quantum theory: quantum field theory |quantum theory: quantum gravity |teleology: teleological arguments for God’s existence

Acknowledgments

I am grateful to Luke Barnes, Friedrich Harbach, Robert Harlander andtwo anonymous referees for helpful comments on earlier versions. Workon this article was supported by the Netherlands Organization forScientific Research (NWO), Veni grant 275-20-065.