The term “naturalism” has no very precise meaning incontemporary philosophy. Its current usage derives from debates inAmerica in the first half of the last century. The self-proclaimed“naturalists” from that period included John Dewey, ErnestNagel, Sidney Hook and Roy Wood Sellars. These philosophers aimed toally philosophy more closely with science. They urged that reality isexhausted by nature, containing nothing “supernatural”,and that the scientific method should be used to investigate all areasof reality, including the “human spirit” (Krikorian 1944,Kim 2003).

So understood, “naturalism” is not a particularlyinformative term as applied to contemporary philosophers. The greatmajority of contemporary philosophers would happily accept naturalismas just characterized—that is, they would both reject“supernatural” entities, and allow that science is apossible route (if not necessarily the only one) to important truthsabout the “human spirit”.

Even so, this entry will not aim to pin down any more informativedefinition of “naturalism”. It would be fruitless to tryto adjudicate some official way of understanding the term. Differentcontemporary philosophers interpret “naturalism”differently. This disagreement about usage is no accident. For betteror worse, “naturalism” is widely viewed as a positiveterm in philosophical circles—only a minority of philosophers nowadaysare happy to announce themselves as“non-naturalists”.^[1] This inevitably leads to a divergence inunderstanding the requirements of “naturalism”. Thosephilosophers with relatively weak naturalist commitments are inclinedto understand “naturalism” in a unrestrictive way, inorder not to disqualify themselves as “naturalists”,while those who uphold stronger naturalist doctrines are happy to setthe bar for “naturalism”higher.^[2]

Rather than getting bogged down in an essentially definitionalissue, this entry will adopt a different strategy. It will outline arange of philosophical commitments of a generally naturalist stamp,and comment on their philosophical cogency. The primary focus will beon whether these commitments should be upheld, rather than on whetherthey are definitive of “naturalism”. The important thingis to articulate and assess the reasoning that has led philosophers ina generally naturalist direction, not to stipulate how far you need totravel along this path before you can count yourself as a paid-up“naturalist”.

As indicated by the above characterization of themid-twentieth-century American movement, naturalism can be separatedinto an ontological and a methodological component. The ontologicalcomponent is concerned with the contents of reality, asserting thatreality has no place for “supernatural” or other“spooky” kinds of entity. By contrast, the methodologicalcomponent is concerned with ways of investigating reality, andclaims some kind of general authority for the scientific method.Correspondingly, this entry will have two main sections, the firstdevoted to ontological naturalism, the second to methodologicalnaturalism.

Of course, naturalist commitments of both ontological andmethodological kinds can be significant in areas other thanphilosophy. The modern history of psychology, biology, social scienceand even physics itself can usefully be seen as hinging on changingattitudes to naturalist ontological principles and naturalistmethodological precepts. This entry, however, will be concerned solelywith naturalist doctrines that are specific to philosophy. So thefirst part of this entry, on ontological naturalism, will be concernedspecifically with views about the general contents of reality that aremotivated by philosophical argument and analysis. And the second part,on methodological naturalism, will focus specifically onmethodological debates that bear on philosophical practice, and inparticular on the relationship between philosophy and science.

• 1. Ontological Naturalism
  • 1.1 Making a Causal Difference
  • 1.2 Modern Science and Causal Influence
  • 1.3 The Rise of Physicalism
  • 1.4 Reductive and Non-Reductive Physicalism
  • 1.5 Physicalist Downwards Causation
  • 1.6 Conscious Properties and Closure Arguments
  • 1.7 Moral Facts
  • 1.8 Mathematical Facts
• 2. Methodological Naturalism
  • 2.1 Philosophy and Science
  • 2.2 The Canberra Plan
  • 2.3 SyntheticA Priori Intuitions?
  • 2.4 A Role for Intuitions
  • 2.5 Mathematical, Modal and Moral Knowledge
• Bibliography
• Academic Tools
• Other Internet Resources
• Related Entries

1. Ontological Naturalism

1.1 Making a Causal Difference

A central thought in ontological naturalism is that allspatiotemporal entities must be identical to or metaphysicallyconstituted by physical^[3] entities. Many ontological naturalists thusadopt a physicalist attitude to mental, biological, social and other such“special” subject matters. They hold that there isnothing more to the mental, biological and social realms thanarrangements of physical entities.

The driving motivation for this kind of ontological naturalism isthe need to explain how special entities can have physical effects.Thus many contemporary thinkers adopt a physicalist view of the mentalrealm because they think that otherwise we will be unable to explainhow mental events can causally influence our bodies and otherphysical items. Similar considerations motivate ontologicallynaturalist views of the biological realm, the social realm, and soon.

It may not be immediately obvious why this need to account forphysical effects should impose any substantial naturalist constraintson some category. After all, there seems nothinga prioriincoherent in the idea of radically unscientific“supernatural” events exerting a causal influence onphysical processes, as is testified by the conceptual cogency oftraditional stories about the worldly interventions of immaterialdeities and other outlandish beings.

However, there may bea posteriori objections to suchnon-natural causal influences on the physical world, even if there arenoa priori objections. We shall see below how modernscientific theory places strong restrictions on the kinds of entitiesthat can have physical effects. Given that mental, biological andsocial phenomena do have such effects, it follows that they mustsatisfy the relevant restrictions.

Note how this kind of argument bites directly only on thosecategories that do have physical effects. It places no immediateconstraints on categories that lack any such effects, which arguablyinclude the mathematical and modal realms, and perhaps the moralrealm. We shall return to the question of whether there are anyfurther reasons for ontologically naturalist views about such putativelynon-efficacious categories in sections1.7and1.8 below.

1.2 Modern Science and Causal Influence

There is an interesting history to modern science’s viewsabout the kinds of things that can produce physical effects (Papineau 2001). It willbe worth rehearsing this history in outline, if only to forestall acommon reaction to ontological naturalism. It is sometimes suggestedthat ontological naturalism rests, not on principled grounds, but onsome kind of unargued commitment, some ultimate decision to nailone’s philosophical colours to the naturalistmast.^[4] Andthis diagnosis seems to be supported by the historical contingency ofontologically naturalist doctrines, and in particular by the fact that they havebecome widely popular only in the past few decades. However,familiarity with the relevant scientific history casts the matter in adifferent light. It turns out that naturalist doctrines, far fromvarying with ephemeral fashion, are closely responsive to receivedscientific opinion about the range of causes that can have physicaleffects.

A short version of this history runs like this: (1) the mechanisticphysics of the seventeenth century allowed only a very narrow range ofsuch causes; (2) early Newtonian physics was more liberal, and indeeddid not impose any real restrictions on possible causes of physicaleffects; (3) however, the discovery of the conservation of energy inthe middle of the nineteenth century limited the range of possiblecauses once more; (4) moreover, twentieth-century physiologicalresearch has arguably provided evidence for yet furtherrestrictions.

Let us now rehearse this story more slowly.

(1) The “mechanical philosophers” of the earlyseventeenth century held that any material body maintains a constantvelocity unless acted on, and moreover held that all action is due toimpact between one material particle and another. So stated, themechanical philosophy immediately precludes anything except impactingmaterial particles from producing physical effects. Leibniz saw thisclearly, and concluded that it discredited Descartes’interactive dualism, which had a non-material mind influencing thephysical world (Woolhouse 1985). (Of course, Leibniz did not therewithreject dualism and embrace the physicalist view that minds arecomposed of material particles, but instead opted for“pre-established harmony”. Views which avoid ontologicalnaturalistic views of the mind by denying that mental events have any physicaleffects will be discussed further insection1.6 below.)

(2) At the end of the seventeenth century Newtonian physicsreplaced the mechanical philosophy of Descartes and Leibniz. Thisreinstated the possibility of interactive dualism, since it allowed thatdisembodied forces, as well as impacts, could cause physicaleffects. Newtonian physics was open-ended about the kinds of forcesthat exist. Early Newtonians posited fundamental mental and vitalforces alongside magnetic, chemical, cohesive, gravitational andimpact forces. Accordingly, they tooksui generis mentalaction in the material world to be perfectly consistent with theprinciple of physics. Moreover, there is nothing in the originalprinciples of Newtonian mechanics to stop mental forces arisingautonomously and spontaneously, in line with common assumptions aboutthe operation of the mind (Papineau 2001: Section 7).

(3) In the middle of the nineteenth century the conservation ofkinetic plus potential energy came to be accepted as a basic principleof physics (Elkana 1974). In itself this does not rule out fundamentalmental or vital forces, for there is no reason why such forces shouldnot be “conservative”, operating in such a way as tocompensate losses of kinetic energy by gains in potential energyandvice versa. (The term “nervous energy” is arelic of the widespread late nineteenth-century assumption that mentalprocesses store up a species of potential energy that is then releasedas kinetic energy in action.) However, the conservation of energy does imply that anysuch special forces must be governed by strict deterministic laws: ifmental or vital forces arose spontaneously, then there would benothing to ensure that they never led to energy increases.

(4) During the course of the twentieth century receivedscientific opinion became even more restrictive about possiblecauses of physical effects, and ruled out anysui generismental or vital causes, even of a law-governed and predictablekind. Detailed physiological research, especially into the operation of nerve cells,gave no indication of any physical effects that cannot be explainedin terms of basic physical forces that also occur outside livingbodies. By the middle of the twentieth century, belief insuigeneris mental or vital forces had become a minority view. Thisled to the widespread acceptance of the doctrine now known as the“causal closure” or the “causal completeness ofthe physical”, according to which all physical effects havefully physical causes.

1.3 The Rise of Physicalism

This historical sequence casts light on the evolution ofontologically naturalist doctrines. In the initial seventeenth-centurymechanical phase, there was a tension, as Leibniz observed, betweenthe dominant strict mechanism and interactive dualism. However, oncemechanism was replaced by a more liberal understanding of forces in the second Newtonian phase, science ceased to raise any objections todualism and more generally to non-physical causes of physical effects.As a result, the default philosophical view was a non-naturalistinteractive pluralism which recognized a wide range of fundamentalnon-physical influences, including spontaneous mental influences (or“determinations of the soul” as they would then have beencalled).

In the third phase, the nineteenth-century discovery of theconservation of energy continued to allow thatsui generisnon-physical forces can interact with the physical world, butrequired that they be governed by strict force laws.Suigeneris mental and vital forces were still widely accepted, butan extensive philosophical debate about the significance of theconservation of energy led to a widespread recognition that any suchforces would need to be law-governed and thus amenable to scientificinvestigation. We might usefully view this as a species ofontological naturalism that falls short of full physicalism. Mentaland other special forces were stillsui generis and non-physical, but even so they fell within the realm ofscientific law and so could not operate spontaneously. (As many commentatorsat the time recognized, this weaker form of naturalism alreadycarried significant philosophical implications, particularly for thepossibility of free will.^[5])

In the final twentieth-century phase, the acceptance of the causalclosure of the physical led to full-fledged physicalism. The causalclosure thesis implied that, if mental and other special causes are toproduce physical effects, they must themselves be physicallyconstituted. It thus gave rise to the strong physicalist doctrine thatanything that has physical effects must itself be physical.

In support of this understanding of the twentieth-century history, it isnoteworthy how philosophical arguments in favour of physicalism began to appear from the 1950s onwards. Some of these arguments appealedexplicitly to the causal closure of the physical realm (Feigl 1958,Oppenheim and Putnam 1958). In other cases, the reliance on causalclosure lay below the surface. However, it is not hard to see thateven in these latter cases the causal closure thesis played a crucialrole.

Thus, for example, consider J.J.C. Smart’s (1959) thoughtthat we should identify mental states with brain states, for otherwisethose mental states would be “nomological danglers” whichplay no role in the explanation of behaviour. Or take DavidLewis’s (1966) and David Armstrong’s (1968) argumentsthat, since mental states are picked out by their causal roles, andsince we know that physical states play these roles, mental statesmust be identical with those physical states. Finally, consider DonaldDavidson’s (1970) argument that, since the only laws governingbehaviour are those connecting behaviour with physical antecedents,mental events can only be causes of behaviour if they are identicalwith those physical antecedents. At first sight, it might not beobvious that these arguments require the causal closure thesis. But amoment’s thought will show that none of these arguments wouldremain cogent if the closure thesis were not assumed, and it were thusleft open that some physical effects (the movement of matter in arms,perhaps, or the firings of the motor neurones which instigate thosemovements) were not determined by prior physical causes at all, butbysui generis mental causes.

Quantum mechanics is often taken to involve a departure fromdeterminism. Sometimes it is suggested that indeterminism of thissort creates room forsui generis non-physical causes toinfluence the physical world. However, even if quantum mechanicsimplies that some physical effects are themselves undetermined, itprovides no reason to doubt a quantum version of the causal closurethesis, to the effect that thechances of those effects arefully fixed by prior physical circumstances. And this alone is enoughto rule outsui generis non-physical causes. For suchsuigeneris causes, if they are to be genuinely efficacious, mustpresumably make an independent difference to the chances of physicaleffects, and this in itself would be inconsistent with the quantumcausal closure claim that such chances are already fixed by priorphysical circumstances. Once more, it seems that anything that makes adifference to the physical realm must itself be physical.

1.4 Reductive and Non-Reductive Physicalism

It will be worth being explicit about the way the causal closureprinciple supports physicalism. First we assume that mental causes(biological, social, …) have physical effects. Then the causalclosure principle tells us that those physical effects have physicalcauses. So, in order to avoid an unacceptable proliferation of causesfor those physical effects (no “systematicoverdetermination”), we need to conclude that the mental(biological, social, …) causes of those effects are notontologically separate from their physical causes.

However, even if this general line of argument is accepted, thereis room for differing views about exactly what its denial ofontological separateness requires. Let us agree that causes are“events” (or “facts”) that involveinstantiations of properties.^[6] So, if some special cause is notontologically separate from some physical cause, the propertyinstantiations that it involves cannot themselves be ontologicallyseparate from the property instantiations involved in the physicalcause. At this point, however, there are divergent views about howtight a constraint this imposes.

One school holds that it requires type-identity, the strictidentity of the relevant special properties with physicalproperties. On the other side stand “non-reductive”physicalists, who hold that the causal efficacy of special causes willbe respected as long as the properties they involve are“realized by” physical properties, even if they are notreductively identified with them.

Type-identity is the most obvious way to ensure thenon-separateness of special and physical causes: if exactly the sameproperties comprise the special and physical cause, the two causeswill themselves be fully identical. Still, type-identity is a verystrong doctrine. Type identity about thoughts, for example, wouldimply that the property of thinking about the square root of two isidentical with some physical property. And this seems highlyimplausible. Even if all human beings with this thought must bedistinguished by some common physical property of theirbrains—which itself seems unlikely—there remains theargument that other life-forms, or intelligent androids, will also beable to think about the square root of two, even though their brainsmay share no significant physical properties with ours (Fodor 1974,Bickle 2013).

This “variable realization” argument has led manyphilosophers to seek an alternative way of reconciling the efficacy ofmental and other special causes with the causal closure thesis, onewhich does not require the strict identity of non-physical andphysical properties. The general idea of this “non-reductivephysicalism” is to allow that instantiations of a given specialproperty will always be grounded in or metaphysically determined byinstantiations of physical properties, but to add that these“realizing” physical properties might be different indifferent cases. So, for example, any being who thinks about thesquare root of two will do so in virtue of instantiating some physicalproperties, but these can be different physical properties indifferent cases—in one human being it may be one set of neuralarrangements, in another a different set, and in other life forms itmight involve nothing like neural properties at all.

There are various more detailed ways of filling out this idea ofnon-reductive physicalism. A common feature is the requirement thatspecial properties should metaphysically supervene on physicalproperties, in the sense that any two beings who share the realizingphysical properties will necessarily share the same specialproperties, even though the physical properties which so realize thespecial ones can be different in different beings. This arguablyensures that nothing more is required for any specific instantiationof a special property than its physical realization—even Godcould not have created your brain states without thereby creating yourfeelings—yet avoids any reductive identification of specialproperties with physical ones. (This is a rough sketch of thesupervenience formulation of physicalism. For more see Stoljar2015.)

Some philosophers object that non-reductive physicalism does not infact satisfy the original motivation for physicalism, on the groundsthat it does not really reconcile the efficacy of mental and otherspecial causes with the causal closure thesis (Kim 1998, Robb and Heil2014: Section 6). According to non-reductive physicalism, specialproperties are not type-identical with any strictly physicalproperties, even though they supervene on them. And this then seems toimply that any given special cause will be distinct from thephysical cause that realizes it, since it involves theinstantiation of a different property. (The property of thinking aboutthe square root of two is definitely a different property from theneural property that realizes it in me, say, since another being couldshare the former property without sharing the latter.)

The opponents of non-reductive physicalism then insist that thisgives us an unacceptable proliferation of causes for the physicaleffects of special causes after all—both the physical causeimplied by the causal closure thesis and the distinct specialcause. In response, advocates of non-reductive physicalism respondthat there is nothing wrong with such an apparent duplication ofcauses if it is also specified that the latter metaphysicallysupervene on the former.

The issue here hinges on the acceptability of different kinds ofsystematic overdetermination (Bennett 2003). All can agree that itwould be absurd if the physical effects of special causes always hadtwo metaphysically independent causes. Plugging this into the causalclosure argument for physicalism, we can conclude that there can beno metaphysically independent non-physical causes (such as Cartesiandualist mental causes) for effects that already have full physicalcauses. However, even if “strong overdetermination” bytwo ontologically independent causes is so ruled out, this does notnecessarily preclude “weak overdetermination” by both aphysical cause and a metaphysically supervenient special cause.Advocates of non-reductive physicalism argue that this kind ofoverdetermination is benign and consistent with the causalargument’s denial of strong overdetermination, since now thetwo causes are not metaphysically distinct—the special causeisn’t genuinely additional to the physical cause (nothing moreis needed for your feelings than your brain states).

1.5 Physicalist Downwards Causation

Some recent writers have explored a different way of upholding thecausal efficacy of non-reduced mental and other special causes. Wherethe “benign overdetermination” option says certain effectshave a special causeas well as a physical cause, thesewriters urge that some effects have a special causeinsteadof a physical cause.

Suppose a pigeon pecks at crimson tiles. Is the pecking caused bythe specific shade, crimson, or the more generic colour, red? Thenatural answer is that it depends. If the pigeon pecks only at crimsontiles, and not at other shades of red, then it is the crimsonness thatis causing the pecking; but if the pigeon pecks at any shade ofred, it is the redness. Examples like these have led a number ofwriters to require that causes beproportional to theireffects (Yablo 1992; Menzies 2008; List and Menzies 2009, 2010). Weshould attribute the effect to that property that is specific enoughto suffice for it, but no more specific than that.

This suggests that sometimes special causes andnot theirphysical realizers might be responsible for physical effects. SupposeI want to hail a taxi, and that this desire is realized by some brainstate, and that I then wave my arm. It seems that it will then be thedesire that is proportional to the waving, not the brain state, inthat I would still have waved my arm if my desire had been differentlyrealized, though not if I hadn’t had the desire at all.

Some will say that in such cases the desirecausallyexplains the waving, but that it is still the brainstate thatcauses it. This thought appeals to the intuitionthat real causal relations are always constituted by basic physicalinteractions, by bits of matter bumping into each other. But thisintuition is not decisive, and a number of theoretical considerationsspeak against it. For instance, the “difference-making”account of causation developed by James Woodward implies that genericproperties often eclipse their more specific realizations ascauses (Woodward 2005), as does the view that causation is arelatively macroscopic phenomenon whose temporal asymmetry isanalogous to the temporal asymmetries of thermodynamics (Loewer 2007,Papineau 2013).

It is worth observing that physicalists who advocate this kind ofdownwards special causation are in some danger of sawing off thebranch they are sitting on, in that they now seem to be advocatingcounter-examples to the causal closure of the physical. If myarm’s waving is caused by my desire and not by my brain state,it would seem to have a mental cause and not a physical cause, and thus run counter tothe closure thesis that every physical effect has some fully physicalcause. Since the original rationale for embracing physicalism wassupposed to be science’s discovery that the physical realm iscausally closed, this may seem to leave physicalists in an awkwardposition.

However, even if the examples of “downwards” causationdo undermine the thesis of the causal closure of the physical, itmay still be possible to rework the original argument forphysicalism in terms of closure under nomological determination,rather than causal closure.^[7] Nothing in the idea of proportionalcausation threatens the idea that modern physics shows that allphysical effects (or their chances) are fully nomologicallydetermined by physical antecedents, even if they aren’t always caused by them. And this in itself argues that metaphysically independent special causes would imply an unacceptable strong overdetermination of physical events.

1.6 Conscious Properties and Closure Arguments

Some philosophers take there to be compelling arguments against theview thatconscious states are metaphysically constituted byphysical states. This rejection of physicalism about conscious properties certainlyhas the backing of intuition. (Don’t zombies—beings whoare physically exactly like humans but have no consciouslife—intuitively seem metaphysically possible?) However,whether this intuition can be parlayed into a sound argument is ahighly controversial issue, and one that lies beyond the scope ofthis entry. A majority of contemporary philosophers probably holdthat physicalism can resist thesearguments.^[8]But a significant minority take the other side.

This minority has a number of options. One is to hold that conscious properties are "epiphenomenal" and do not exert any influence on brain processes or subsequent behaviour (Jackson 1982, 1986; Chalmers1996).^[9] Another is to embrace the "overdeterminationist" view that the physical results of conscious causes are always strongly overdetermined , both by their normal physical antecedents and by the metaphysically independent conscious causes (Mellor 1995, Kroedel 2015). Still, neither of these positions is attractive. They require us to posit odd causal structures, either involving a species of effects that are never themselves causes, or a species of effects that are always strongly overdetermined. Given that nature displays no other examples of such causal structures, general principles of theory choice would seem to argue against both epiphenomenalism and overdeterminationism.

In consequence, a yet further option has become increasingly popular among those who are persuaded that consciousness must transcend the physical realm. This is the Russellian monist option that locates conscious properties among the basic categorical properties that play the dispositional roles described by physical science. This option has the virtues of separating consciousness from the world described by physics without positing any special causal structures operating in the brain (Alter and Pereboom 2019).

It is an interesting question whether Russellian monism is necessarily opposed to physicalism rather than a special case of it (Montero 2015, Brown 2017). In any case, we should note that Russellian monism is designed to conform to the causal (or determinational) closure of the physical realm, as indeed are epiphenomenalism and overdeterminationism. It is striking that nearly all contemporary views of the mind-brain relations are naturalist at least to the extent that they respect this closure thesis. Strongly interactionist views that allow the conscious mind to make an independent difference to the physical world have few defenders nowadays (but see Lowe 2000, 2003; Steward 2015).

1.7 Moral Facts

G.E. Moore’s well-known “open question” argumentis designed to show that moral facts cannot possibly be identical tonatural facts. Suppose that the natural properties of some situationare completely specified. It will always remain an open question,argued Moore, whether that situation is morally good or bad (Moore1903).

Moore took this argument to show that moral facts constitute adistinct species of non-natural fact. However, any such non-naturalistview of morality faces immediate difficulties, deriving ultimatelyfrom the kind of causal closure thesis discussed above. If allphysical effects are due to a limited range of physically-groundednatural causes, and if moral facts lie outside this range, then itfollow that moral facts can never make any difference to what happensin the physical world (Harman 1986). At first sight this may seemtolerable (perhaps moral facts indeed don’t have any physicaleffects). But it has awkward epistemological consequences. For beingslike us, knowledge of the spatiotemporal world is mediated by physicalprocesses involving our sense organs and cognitive systems. If moralfacts cannot influence the physical world, then it is hard to see howwe can have any knowledge of them.

The traditional non-naturalist answer to this problem is to posit anon-natural faculty of “moral intuition” that gives ussome kind of direct access to the moral realm (as explained in Ridge2014: Section 3). However, causal closure once more makes itdifficult to make good sense of this suggestion. Presumably at somepoint the posited intuitive faculty will need to make a causaldifference in the physical world (by affecting what people say anddo, for example). And at this point the causal closure argument willbite once more, to show that a non-natural intuitive faculty would implausibly imply that some of our actions are strongly overdetermined by two metaphysically independent antecedents.

Moral non-naturalism has had something of a revival in recent years, with defenders including Russ Shaffer-Landau (2003), Ralph Wedgwood (2007), Derek Parfit (2011) and David Enoch (2011). Still, the challenge of accounting for our access to non-natural moral facts remains, and it is debatable whether any of these writers has found a satisfactory alternative to a causally problematic faculty of intuition. Perhaps the most developed suggestion is Enoch’s (2011) appeal to the indispensability of non-natural moral facts to moral reasoning, a line of argument that is analogous to Hilary Putnam’s case for non-natural mathematical objects, to be discussed in the next section below. But Enoch’s appeal arguably faces many of the same general objections as Putnam’s argument, as well as objections specific to the moral realm (see Leng 2016).

In light of the difficulties facing moral non-naturalism, mostcontemporary moral philosophers opt instead for some species ofnaturalist view. We can divide the naturalist options here into twobroad categories: irrealist and realist. Irrealist moral naturalistsaim to account for moral discourse by offering naturalist accounts ofthe social and linguistic and practices that govern it, but withoutsupposing that moral utterances report on moral facts with asubstantial independent existence (Joyce 2015). By contrast,naturalist moral realists agree with moral non-naturalists thatsubstantial moral facts exist, but seek to locate them in the naturalrealm rather than in somesui generis non-natural realm(Lenman 2014).

Both these broad categories have further sub-divisions. Among theirrealists, we can distinguish explicitly non-cognitivist views likeemotivism and prescriptivism which deny that moral judgements expressbeliefs (Hare 1952, Blackburn 1993, Gibbard 2003) from cognitivistviews that accept that moral judgements do express beliefs but deny asubstantial reality to the putative facts to which they answer; andamong the latter cognitivist views we can distinguish error-theoreticfictionalist options which view moral judgements as simply false(Mackie 1977, Kalderon 2005) from projectivist options which hold thatmoral discourse is sufficiently disciplined for its judgements toqualify for a species of truth even though they do not report onindependently existing causally significant facts (Wright 1992, Price2011).

Naturalist moral realism also comes in different varieties. Inrecent debates two versions have figured prominently; “Cornellrealism”, which includes moral facts among the causallysignificant facts but resists their type-reducibility to non-moralfacts (Sturgeon 1985, Boyd 1988), and “moralfunctionalism” which is happy to equate moral facts withstraightforwardly descriptive facts (Jackson 1998).

Any kind of moral naturalist realist needs to reject Moore’sopen question argument. There are two alternatives here. One is toinsist that Moore’s posited openness is relatively superficial,and that there is no principled barrier to inferring moral factsapriori from the non-moral natural facts, even if such inferenceswill sometimes require a significant amount of information and reflection. Theother is to argue that the constitution of moral facts by non-moralnatural facts is ana posteriori matter, akin to the relationbetween water and H₂O, and that therefore Moore’sopenness only points to a conceptual gap, not a metaphysical one(Ridge 2014: Section 2).

This sub-section has focused on morality. But there are othertopics which arguably involve matters of value, such as aesthetics, the normativity of theoretical and practical reason, and so on. The constraints placedon theories of moral facts by naturalist considerations willapply,mutatis mutandis, in these areas too, militatingagainst theories that posit non-natural value-bearing facts and infavour of naturalist alternatives, of either a realist or irrealiststripe.

1.8 Mathematical Facts

Mathematics raises many of the same issues for ontologicalnaturalism as morality. Mathematical claims typically involve acommitment to abstract objects like numbers and sets, eternal entitiesoutside space and time. This might seem cogent at first sight, butonce more epistemological difficulties quickly arise. Abstract objectscan have no effects in the spatiotemporal world. How then canspatiotemporal being like ourselves come to know about them?

However the mathematical case does not fully parallel the moralone. One of the options in the moral case was naturalist realism,which reads moral claims as about natural facts which play causalroles in the spatiotemporal world. However, given the explicitcommitment of mathematical claims to abstract objects withoutspatiotemporal location, this option does not seem available in themathematical case (but see Maddy 1990). So we seem required to choosebetween non-naturalist realism about non-spatiotemporal mathematicalentities or naturalist irrealism.

As in the moral case, non-naturalist realism faces epistemologicalchallenges, to which one response is to posit afaculty of intuition which gives us access to the abstractmathematical realm. Kurt Gödel arguably favoured a view alongthese lines (Parsons 1995). However, once more this only seems to pushthe problem back. There seems no good way for the posited faculty tobridge the causal gap between the abstract and the spatiotemporalrealms without generating implausible overdetermination.

An alternative version of non-naturalist realism aims to vindicatemathematical and modal claims as essential parts of our best overalltheories of the world. According to this line of thought, defended byHilary Putnam, our empirically best-supported scientific theoriescommit us to mathematical entities; ergo, we are entitled to believein such entities (Putnam 1971).^[10]

However, it is contentious whether our best-supported empiricaltheories do commit us to abstract mathematical entities. The mostprominent version of naturalist irrealism about mathematics, HartryField’s fictionalism, disputes precisely this claim. Accordingto Field, we can construct “nominalist”versions of scientific theories that avoid commitment to abstractmathematical objects yet are explanatorilysuperior to the “platonist” alternatives. Field arguesthat we do not have to regard mathematics itself as literally true inorder to understand its use in science and other applications. Ratherit can be viewed as a “useful fiction” which facilitatesinferences between nominalistic scientific claims, but is not itselfimplicated in our most serious beliefs about the world (Field 1980,1989).

Not all philosophers of mathematics are convinced that Fieldiannominalizations are available to replace all scientific references toabstract mathematical objects. In particular, some have argued thatcertain explanations of nominalist facts make essential reference toabstract objects (Baker 2005, Batterman 2010). In response, othershave sought to show that there are in fact good nominalistexplanations of the facts in question (Daly and Langford 2009,Butterfield 2011, Menon and Callender 2013). In any case, it is notclear that Field’s metaphysical stance requires a full executionof his nominalizing programme, as opposed to a case for its cogency:difficulties in constructing nominalist theories can always beattributed to limitations of human ingenuity rather than the realityof abstract mathematical objects (Leng 2013).

Perhaps the most popular contemporary alternative to fictionalismis the version of non-naturalist realism offered by the neo-Fregeanthesis that abstract mathematical beliefs can be justified as analytictruths that follow from logic and certain meaning stipulations. Theidea has been most fully developed in connection with arithmetic,where Crispin Wright has shown how Peano’s postulates can bederived within the framework of second-order logic from nothing exceptthe Humean principle that the same number attaches to equinumerousconcepts. According to Wright, this principle can be viewed as animplicit definition of our concept of number. If this is right, thenit has indeed been shown that arithmetic, and therewith the existenceof numbers as abstract objects, follows from logic and definitionalone (Wright 1983, Hale and Wright 2003). There has also been someattempt to extend the programme to mathematical analysis (Shapiro2000, Wright 2000).

One query that might be put to this programme is whether the Humeanprinciple and analogous assumptions can really be viewed as analyticdefinitions. If they commit us to numbers and other abstract objectswhose existence cannot established without them, then they arearguably doing more than definitions should. A related issue iswhether the overall neo-Fregean position is properly viewed asrealist. From its perspective, the role of abstract mathematicalobjects in the overall scheme of things seems to be exhausted by theirmaking our mathematical statements true; given this, it might seembetter to classify it as a species of irrealism (MacBride 2003).

To complete this discussion of ontological naturalism, let usbriefly consider the realm of modality, understood as the subjectmatter of claims that answer to something more thanactuality. Modality raises many of the same issues as mathematics, butthe topic is complicated by the prior question of the content of modalclaims, and in particular about whether they quantify over non-actualpossible worlds. Whereas there is little dispute about the initialsemantic analysis of mathematical claims—they purport to referto abstract numbers, sets, functions and so on—there is somewhatless unanimity about the possible worlds analysis of modal claims(Nolan 2011b).

To the extent that modal claims do quantify over possible worlds,the ontological points made about mathematics apply well here too.Since non-actual worlds do not inhabit our spatiotemporal realm, anontologically naturalist realism seems to be ruled out from thestart. The remaining alternatives are irrealism or non-naturalistrealism. The former alternative has been explored in recent years bymodal fictionalists (Rosen 1990; Nolan 2011a). The options underthe latter heading meet the same epistemological challenges as inthe mathematical case: brute intuition faces causal problems; it iscontentious whether we should take our best scientific theories tocommit us to possible worlds; and, if modal knowledge is to beanalyticallya priori, on the model of mathematicalneo-Fregeanism, then it is not obvious that it can take us toknowledge of possible worlds construed realistically.

2. Methodological Naturalism

2.1 Philosophy and Science

In what follows, “methodological naturalism” will beunderstood as a view about philosophical practice. Methodologicalnaturalists see philosophy and science as engaged in essentially thesame enterprise, pursuing similar ends and using similar methods.

Among philosophers of religion, “methodologicalnaturalism” is sometimes understood differently, as a thesis about naturalscientific method itself, not about philosophical method. In thissense, “methodological naturalism” is the view that religiouscommitments have no relevance within science: natural science itselfrequires no specific attitude to religion, and can be practised justas well by adherents of religious faiths as by atheists or agnostics(Draper 2005). This thesis is of interest to philosophers of religionbecause many of them want to deny that methodological naturalism inthis sense entails “philosophical naturalism”, understoodas atheism or agnosticism. You can practice natural science in justthe same way as non-believers, so this line of thought goes, yetremain a believer when it comes to religious questions. Not all defenders of religious belief endorse this kind of“methodological naturalism”, however. Some think that religiousdoctrines do make a difference to scientific practice, yet aredefensible for all that (Plantinga 1996). In any case, this kind of“methodological naturalism” will not be discussed furtherhere. Our focus will be on the relation between philosophy andscience, not between religion and science.

What exactly is involved in adopting a methodologically naturalist attitude to the relation between philosophy and science? In order to focus what follows, let us understand methodological naturalism as the specific claim that philosophy and science are both concerned to establishsynthetic knowledge about the natural world, and moreover to achievethis bya posteriori investigation.

Methodological naturalists will of course allow thatthere are some differences between philosophy and science. But theywill say that these are relatively superficial, a matter of focusing on differentquestions rather than any radical difference of approach. For one thing, philosophical questions are oftendistinguished by their great generality. Where scientists think aboutviruses, electrons or stars, philosophers think about spatiotemporalcontinuants, properties, causation or time, categories thatstructure all our thinking about the natural world. Another common feature of philosophical questions is that they involve some kind oftheoretical tangle. Our thinking supports different lines of thought that leadto conflicting conclusions. Progress requires an unravelling ofpremises, including perhaps an unearthing of implicit assumptionsthat we didn’t realize we had.

For both these reasons, philosophical issues are rarely resolved by new observational data. The normal philosophical predicament is that we have all the observational data we could want, but aren’t sure of the best way of accommodating them. Still, methodological naturalists will urge, this doesn’t mean that a posteriori synthetic theories are not the aim of philosophy. A theory produced by unravelling the theoretical tangles surrounding some general category can still be an a posteriori synthetic theory, even if no new observational findings went into its construction.

From the methodological naturalist perspective, then, philosophical views are synthetic claims that answer to the overall tribunal of a posteriori observational evidence. The obvious objection to this view, however, is that it does not accord with philosophical practice. In particular, it seems in tension with the central role thatintuitions play in philosophy. The typical way to assess philosophical views is arguably to test them against intuitive judgements about possible cases, not againsta posteriori observational data. So, for example, the description theory of names is challenged by our intuitions about Kripke’s imagined counterexamples, the tripartite theory of knowledge by our intuitive reactions to Gettier cases, and so on. At first pass, this certainly suggests that philosophy is centrally concerned with the analysis of everyday concepts, not the construction of synthetic theories: it is using intuitions about possible cases to uncover the structure implicit in our concepts. The reliance on intuitions thus argues that, far from delivering synthetica posteriori knowledge, philosophy uses a priori methods to deliver analytic conclusions.

Methodological naturalists can respond to this challenge at a number of points. For one thing, they can ask whether intuitions really do play a central role in philosophy. For another, they can query whether, even if they do, they are reallya priori intuitions. And finally, they can dispute whether, even if the intuitions area priori, they are genuinely analytic. It will be convenient to consider these different responses in reverse order.

2.2 The Canberra Plan

One influential strand in contemporary philosophy is quite explicit in claiming that analytic intuitions play a central role in philosophy. Inspired by David Lewis, and led by Frank Jackson and David Chalmers, this is widely known as the “Canberra Plan”. On this conception, philosophy starts with an initial analysisof concepts employed by everyday thought, such asfree will,say, orknowledge, ormoral value, orconsciousexperience. Once this stage has been completed, philosophycan then turn to “serious metaphysics” to demonstrate howa limited number of ingredients (for example, physical ingredients)might satisfy these everyday concepts. This second stage is likely toappeal to synthetica posteriori scientific knowledge aboutthe fundamental nature of reality. But the purely analytic firststage, according to the Canberra plan, also plays an essential part in setting theagenda for the subsequent metaphysical investigation.(Jackson 1998, Braddon-Mitchell and Nola 2009, Chalmers 2012.)

An initial question about this programme relates to its scope. Itis by no means clear that all, or indeed any, philosophicallyinteresting concepts can be subject to the relevant kind ofanalysis. Jackson himself assumes that pretty much all everydayconcepts can be analysed as equivalent to “the kind whichsatisfies such-and-such folkassumptions”.^[11] But it is arguable that many everydayconcepts are not so constituted, but rather have their semanticcontents fixed by observational, causal or historical relations totheir referents.

Still, we can let this point pass. Even if we suppose, for the sakeof the argument, that a range of philosophically interesting everydayconcepts do have their contents fixed in the way the Canberraprogramme supposes, there are further objections to its understandingof philosophical method.

Let us look a bit more closely at the posited initialagenda-setting stage of the Canberra programme. Upon closerexamination, it is not clear that this makes any essential appeal toanalytic knowledge. Defenders of the Canberra plancharacteristically explain their strategy in terms of “Ramseysentences” (e.g., Jackson 1998: 140). Suppose \(T(F)\) is theset of relevant everyday assumptions involving some philosophicallyinteresting concept. For example,F may be the conceptbelief, and the assumptions inT may include“characteristically caused by perceptions”,“combines with desires to generate actions”, and“has causally significant internal structure”. Then theRamsey sentence corresponding to \(T(F)\) is “\(\exists!\Phi(T(\Phi))\)”.^[12] For the concept belief, this would say: there is some unique kind that is characteristically caused by perceptions, combines with desires to generate actions, and has causally significant internal structure.

The Canberra suggestion is then that, once wehave articulated the relevant Ramsey sentence, we will be in aposition to turn to serious metaphysics to identify the underlyingnature of the F that plays the relevant role.

However, if this is the Canberra procedure, then there is no reasonto think of it as appealing to analytic knowledge at any stage. ARamsey sentence of the kind at issue says that there actually existssome entity satisfying certain requirements (there is a kind of statethat is caused by perceptions …). Sentences like this makeeminently synthetic and falsifiable claims. It is not a matter ofdefinition that humans actually have internal states that play thecausal role associated with the concept of belief. The Canberrastrategy thus seems no different from the prescription that philosophyshould start with the synthetic theories endorsed by everyday thought,and then look to our more fundamental theories of reality to see what,if anything, makes these everyday theories true. This seems entirelyin accord with methodological naturalism—philosophy is in thebusiness of assessing and developing synthetic theories of theworld.

The crucial point here is that Ramsey sentences don’t defineconcepts like belief, but eliminate them. They give us a way of sayingwhat our everyday theories say without using the relevant concept(there is a kind of state that is caused by perceptions …) Ifwe want definitions, then we need “Carnap sentences”, notRamsey sentences (Lewis 1970). The Carnap sentence corresponding to“\(\exists!\Phi(T(\Phi))\)” is “If\(\exists!\Phi(T(\Phi)\), then \(T(F)\)”. (If there isa kind of state that is caused by perceptions …,thenit’s belief.) Carnap sentences can plausibly be viewed as akinto stipulations that fix the reference of the relevant concepts, andto that extent as analytic claims that can be knownapriori. But this certainly does not mean that Ramsey sentences,which make substantial claims about the actual world, are alsoknowable viaa priori analysis. (Note how you can accept aconditional Carnap sentence even if you reject the correspondingunconditional Ramsey sentence. You can grasp the folk concept ofbelief even if you reject the substantial folk theory of belief.)

Can’t defenders of the Canberra programme argue that it isthe analytic Carnap sentences that are crucial in settingphilosophical agendas, not the synthetic Ramsey sentences? But thisseems wrong. We will want to know about the fundamental nature ofbelief if we suppose that there is a kind of state that ischaracteristically caused by perceptions, and so on. That is certainlya good motivation for figuring out whether and how the fundamentalcomponents of reality might constitute this state. But the mere factthat everyday thought contains a concept of such a state in itselfprovides no motivation for further investigation. (In effect, thefunction of a Carnap sentence is to provide a shorthand for talkingabout the putative state posited by the corresponding Ramseysentence. It is hard to see how any important philosophical issuescould hang on the availability of such a shorthand.)

To emphasize the point, consider the everyday concept of a soul,understood as something that is present in conscious beings and survivesdeath. This concept of a soul can be captured by the analytic Carnapsentence: “If certain entities inhabit conscious beings andsurvive death, then they are souls”. Accordingly, this Carnapsentence will be agreed by everybody who has the concept of soul,whether or not they believe in souls. Yet this Carnap sentence willnotper se raise any interesting metaphysical questions forthose who deny the existence of souls. These deniers won’t startwondering how the fundamental constituents of reality realizesouls—after all, they don’t believe in souls. It is onlythose who accept the corresponding Ramsey sentence (“There areparts of conscious beings that survive death”) who will see ametaphysical issue here. Moreover, the Ramsey sentence will pose thismetaphysical issue whether or not it is accompanied by some analyticCarnap sentence to provide some shorthand alternative terminology. Inshort, the methodological naturalist can insist that anybodyinterested in “serious metaphysics” should start byarticulating the substantial existential commitments of our folktheories, as articulated in their synthetic Ramsey sentences. Anyfurther analytic conceptual commitments add nothing of philosophicalsignificance.

The point generalizes beyond the contrast between Ramsey and Carnapsentences. On reflection, it is hard to see why any purelydefinitional analytic truths should matter to philosophy. Syntheticeveryday truisms can certainly be philosophically significant, and sotheir articulation and evaluation can play an important philosophicalrole. But there is no obvious motive for philosophy to concern itselfwith definitions that carry no implications about the contents ofreality.

2.3 SyntheticA Priori Intuitions?

It is worth noting that not all philosophers who advertise themselves as engaging in “conceptualanalysis” are committed to the idea that this involves analytic a priori knowledge. In many cases philosophers who describe themselves in this way go on to explain that in their view “conceptualanalysis” is a matter of articulating synthetic claims and assessing them against a posteriori evidence. A particularly clear version of this picture of conceptual analysis is offered by Robert Brandom (2001). Similarly, recent advocates of “conceptual engineering” are explicit that in their view “concepts” embody substantial commitments which are open to criticism on a posteriori grounds (Cappelen 2018, Cappelen, Plunket and Burgess 2019). In the end, it is not clear what differentiates “conceptual analysis” in this sense from the a posteriori assessment of theories. In any case, this species of conceptual analysis seems perfectly consistent with methodological naturalism. (See also Goldman 2007.)

Other philosophers are also explicit that “conceptualanalysis” issues in synthetic claims, but simultaneously regardit as a source ofa priori knowledge (e.g. Jenkins 2008,2012). This combination of views is less straightforward. Inparticular, it seems open to the traditional query: how is suchsynthetica priori knowledge possible? If some claim is notguaranteed by the structure of our concepts, but answers to the natureof the world, then how is it possible to know it withoutaposteriori evidence?

Yet other philosophers distance themselves from talk of conceptualanalysis, but even so feel that philosophical reflection is a sourceof synthetica priori intuition (Sosa 1998, 2007). They toowould seem to face the traditional query of how syntheticapriori knowledge is possible.

In this context, Timothy Williamson has recently argued that thetraditional distinction betweena priori andaposteriori knowledge is less than clear-cut, and in particularthat it breaks down in connection with the intuitions on whichphilosophers rely. In Williamson’s view, there is a distinctivephilosophical method in which intuitive judgements play a centralrole, but there is no warrant for classifying the relevant intuitionsasa priori rather thana posteriori (Williamson2013).

However, it is arguable that this does not so much address asby-pass the underlying question. Perhaps philosophical intuitions arenot best classified as clearly “a priori”. But,if philosophy’s distinctive methodology relies on syntheticintuitions, this still seems to call for some explanation of their reliability.

Doubts about the reliability of philosophical intuitions have beenamplified over the past few years by the findings of“experimental philosophy”. Empirical studies haveindicated that many central philosophical intuitions are by no meansuniversal, but rather peculiar to certain cultures, social classes andgenders (Knobe and Nichols 2008, 2017). This variability of intuitions is inobvious tension with their reliability. If different people haveopposed philosophical intuitions, then it cannot be that intuitions ofthis kind are always true.

Timothy Williamson has responded to this challenge fromexperimental philosophy by suggesting that, while the intuitions ofordinary people on philosophical matters might be unreliable, those ofphilosophers in particular can be trusted. In his view, a properphilosophical training winnows out mistaken philosophical reactions(2007: 191; 2011). However, this position still seems to call for apositive explanation of how synthetic philosophical knowledge might beestablished withouta posteriori evidence, even if it isrestricted to trained philosophers.

The possibility of such an explanation is not of course to bedismissed out of hand. There is no contradiction inthe idea of experience-independent access to synthetic truths. Afterall, until the eighteenth century no modern philosopher doubted thatGod had bestowed on us powers of reason that would enable us toarrive at perception-independent knowledge of a range of syntheticclaims. Even if few contemporary philosophers would still appeal toGod in this context, there are other possible mechanisms that couldplay an equivalent role.^[13] It is not to be ruled out that ourbiological heritage, for example, has fixed a range of beliefs in uswhose reliability owes nothing to our individual ontogeneticexperience. Indeed there is a case for viewing certain aspects ofour cultural heritage as playing a similar role, imbuing us withcertain beliefs whose justification rests on ancestral rather thanindividual experience.^[14]

However, it is one thing to point to the general possibility ofbiological and cultural mechanisms constitutingexperience-independent sources of reliable knowledge, another toshow that such mechanisms operate within philosophy. Even if thereare areas of thought which rest on such foundations, this does notshow that the intuitions of philosophers in particular have asimilar backing. Moreover, there is arguably direct reason to doubtthat they do. It is not just the intuitions of everyday people onphilosophical matters that have a poor track record. The same appliesto philosophers through history. It is not hard to think ofdeep-seated intuitions appealed to by past philosophers that havesince been discredited. (A purely mechanical being cannot reason;space must be Euclidean; an effect cannot be greater than its cause;every event is determined; temporal succession cannot be relative.)In the next section we will see that there is reason to suppose thatthis unreliability is intrinsic to the nature of philosophy.

2.4 A Role for Intuitions

The challenge to philosophical intuitions is clear enough. Eitherthey are analytic, in which they contain no substantial information,or they are synthetic, in which case they are of dubiousreliability.^[15] Given this, a number of philosopher ofnaturalist inclinations advocate a revisionary attitude tophilosophical method. Philosophy should turn away from intuitions, andinstead engage directly with proper observational evidence. (Kornblith2002, Knobe and Nichols 2008.)

However, this is not the only possible reaction. We would do well to remember that intuitions arguably plays a role in science as well as philosophy. The history of science displays a number of important thought experiments, like Galileo’s analysis of free fall, or the Einstein’s argument against the completeness of quantum mechanics. And intuition functions here very much as it does in philosophy. The scientist imagines some possible situation, and then makes an intuitive judgement about what would happen.

In such cases, it is clearly not mere analytic definitions that are at issue. An eminently synthetic claim—say, that heavier bodies fall faster—is undermined by a contrary intuition—if a big and small body are tied together, they will be heavier than the big one, but will not fall faster. This thought is clearly not guaranteed by concepts alone, but by synthetic assumptions about the way the world works.

Still, this kind of example still faces the second horn of the dilemma about intuitions. If the intuitions involve substantial synthetic claims, why should we take them to be reliable? The historical track record of intuitions is scarcely any better in science than in philosophy. In both realms it would seem wiser to avoid unreliable intuitions and engage directly with observational evidence.

However, there is a way of understanding the role of intuitions in both science and philosophy that evades this worry. Instead of viewing them as designed to elicit authoritative judgments to which philosophical theories must defer, they can instead be seen as deviceswhich help us to articulate our implicit assumptions when we arethreatened with paradox and have difficulty finding a solution.

Recall a point made at the beginning of our discussion ofmethodological naturalism. Philosophical problems are typicallyoccasioned by some kind of theoretical tangle. Different but equallyplausible lines of though lead us to conflicting conclusions.Unraveling this tangle requires that we lay out different theoreticalcommitments and see what might be rejected or modified. A usefulheuristic for this purpose may well be to use intuitions aboutimaginary cases to uncover the implicit synthetic assumptions that areshaping our thinking. This can help us better to appreciate ouroverall theoretical alternatives, and assess which gives the bestoverall fit with thea posteriori evidence.

This perspective on philosophical thought experiments shows why weshould positively expect many of the intuitions they elicit to provewanting. Perhaps there are contexts outside philosophy where variouskinds ofa priori intuitions can be relied upon. But ifphilosophical problems typically arise because we are unsure aboutwhat exactly is amiss in the overall set of synthetic claims we bringto the world, then it would seem only to be expected that the faultwill often lie in the implicit intuitions driving our judgements about cases.

It is worth noting that this often happens with scientific thoughtexperiments too. Galileo’s intuition that light bodies fall asfast as heavy ones was vindicated by subsequent physics. But theverdict can also go the other way. For example, the assumption behindthe Einstein’s argument against the completeness of quantummechanics is nowadays rejected. But this certainly did not mean thathis thought experiment was worthless. On the contrary, it led J. S.Bell to the derivation of the eponymous inequality whose experimentalconfirmation ruled out local hidden variable theories.

It is not hard to think of similar philosophical cases. The worthof philosophical thought-experiments does not always require that theintuitions they elicit are sound. Consider the classic Lockean set-upwhere someone’s memories are transferred to a new body. We allhave an intuition that the person goes with the memories, not the oldbody, as evidenced by our reactions to the many fictions which tradeon just this kind of scenario. But few philosophers of personalidentity would nowadays hold that this intuition is decisive in favourof Lockeanism. Again, consider the intuition that conscious propertiesare ontologically distinct from physical ones, as displayed in ourimmediate reaction to zombie scenarios. Here too, few would supposethat these intuitions by themselves decide the case. Still, even thosewho reject Lockeanism and dualism will allow that reflection onmemory-switching and zombie cases has played a crucial role inclarifying what is at issue in the debates. The evocation ofintuitions by philosophical thought experiments is important, notbecause they provide some special kind ofa priori evidence,but simply because they need to be made explicit and assessed againstthe overalla posteriori evidence.^[16]

This perspective on thought experiments shows that there is a sensein which recent developments within “experimentalphilosophy” can be viewed as complementing traditional armchairmethods. In the previous section we saw that some of the findings ofexperimental philosophy carry the implication that everyday intuitionsare not generally reliable. But in addition to this“negative” message, there is also room for experimentalphilosophy to make a positive philosophical contribution, even incases where there is no variation in intuitions.

Careful experimental probing can helpfully augment traditionalarmchair methods as a way of identifying the structure of implicitassumptions that drive intuitive judgments about test cases.Sometimes thought experiments may be enough. But in more complicatedcases systematic questionnaires and surveys may well be a better wayof identifying the implicit cognitive structures behind ourphilosophical reactions.

Note that experimental philosophy, even when viewed in thispositive light, is at most an addition to our philosophical armoury,not a new way of doing philosophy. For once we have sorted out theintuitive principles behind our philosophical judgements, whether byarmchair reflection or empirical surveys, we still need to assesstheir worth. Even if the claim that we think a certain way issupported by hard empirical data, this doesn’t make that wayof thinking correct. That can only be shown by subjecting that wayof thinking itself to propera posteriori evaluation.

2.5 Mathematical, Modal and Moral Knowledge

Methodological naturalism fits more naturally with some areas ofphilosophy than others. It is perhaps not hard to understand, at leastin outline, how work in areas like metaphysics, philosophy of mind,meta-ethics and epistemology might be aimed at the construction ofsynthetic theories supported bya posteriori evidence. But inother philosophical areas the methodologically naturalistic projectmay seem less obviously applicable. In particular it might be unclearhow it applies to those areas of philosophy that make claims aboutmathematics, first-order morality or modality.

One possibility would be for methodological naturalists to makeexceptions for these areas of philosophy. It would still be asignificant thesis if methodological naturalism could be shown toapply to a number of central areas of philosophy, even if some specialistareas call for a different methodology.

This final subsection will address two issues raised by this suggestion. The first relates specifically to the idea that modal claims constitute a specialism within philosophy. Perhaps mathematical investigation and even first-order moralizing can be regarded as specialist subfields within philosophy. But it is arguable that a concern with the modal realm runs through all of philosophy. If so, then a challenge to the naturalist status of modal claims will threaten the naturalist status of all philosophy.

The second issue will be more directly about the methodologically naturalist status of the three areas mentioned—including mathematics and morality, along with modality. How far do methodological naturalists need to allow that mathematics, morality and modality constitute exceptions to their position in the first place?

On the first issue, Bertrand Russell said

[A philosophical proposition] must not dealspecially with things on the surface of the earth, or with the solarsystem, or with any other portion of space and time. … Aphilosophical proposition must be applicable to everything that existsor may exist. (1917: 110)

However, one can agree with Russell thatphilosophy automatically has implications for the modal realm(“everything that … may exist”), without acceptingthat the aim of philosophy is to explore the modal realm as such. Weneed to distinguish here between an interest in claims which, as ithappens, have modal implications, on the one hand, and an interest inthose modal implications themselves, on the other. It isuncontentious that most of the claims of interest to philosopher havemodal implications. But it does not follow from this that most ofphilosophy is interested in the modal realm itself.

Philosophy is largely concerned with claims about identity andconstitution, claims which as it happens will be necessary if they aretrue. When philosophers ask about knowledge, names, persons,persisting objects, free will, causation, and so on, they are seekingto understand the identity or constitution of these kinds. They wantto know whether knowledge is the same as true justified belief,whether persisting objects are composed of temporal parts, and soon. And so any truths they might establish about such matters willinevitably be necessary rather than contingent, and so carryimplications about a realm beyond the actual.

But the fact thatp impliesnecessarily p doesnot mean that anyone who is interested in the former must beinterested in the latter, any more than someone who is interested inJohn’s age being 47 must be interested in its being a primenumber.

This makes room for methodological naturalists to insist that mostprimary philosophical concerns are synthetic andaposteriori, even if they imply additional modal claims which arenot. Natural science provides a good analogy here. Water isH₂O. Heat is molecular motion. Stars are made of hotgas. Halley’s comet is made of rock and ice. Since all theseclaims concern matters of identity and constitution, they too arenecessary if true. But science is interested in these syntheticaposteriori claims as such, rather than their modalimplications. Chemistry is interested in the composition of actualwater, and not with what happens in other possibleworlds. Methodological naturalists can take the same line withphilosophical claims. Their focus is on whether knowledge is actuallythe same as true justified belief, or whether persisting objects areactually composed of temporal parts—issues which they take to besynthetic anda posteriori—and not with whether thesetruths are necessary—issues which may well have a differentstatus.

Let us now turn to the second issue flagged above. How far domethodological naturalists in fact need to allow thatmodality—and mathematics and first-order morality—do havea different status from the synthetica posteriori characterthey attribute to philosophy in general?

The issues here are by no means clear-cut. Insections1.7 and1.8 abovewe saw how the arguments for ontological naturalism placed generalconstraints on the epistemological options in these area. By and large, these constraints tend to favour naturalism about philosophical method. There is noquestion of exploring these epistemological issues fully here, butsome brief comments will be in order.

For mathematics and modality, the epistemological possibilitieswere restricted to irrealism and ontologically non-naturalistrealism. In the moral case, there were again irrealist options, andalso ontologically naturalist realisms that identified moral factswith causally significant spatiotemporal facts.

For those who endorse irrealist options in any of these areas,there would seem to be no tension with methodologicalnaturalism. After all, irrealist analyses deny that there is anysubstantial knowledge to be had in mathematics, modality or morality,and so will not think of object-level claims in these areas asthemselves contributing to philosophy. (This is consistent withthinking that a meta-understanding of the workings of mathematical,modal or moral discourse is important to philosophy; but then thereseems no reason why such a meta-understanding should be problematicfor methodological naturalism.)

Similarly, there seems no reason why the naturalist realist optionsin the moral case should be in tension with methodological naturalism.The details deserve to be worked through, but on the face of things wemight expect knowledge of causally significant spatiotemporal moralfacts to be synthetic anda posteriori.

This leaves us with non-naturalist realist accounts of mathematicaland modal knowledge. As we saw earlier, the best options here appealto the neo-Fregean programme of grounding knowledge of themathematical and modal realms ina priori analyticprinciples. If this programme could be vindicated, then it wouldindeed violate the requirements of methodological naturalism. But, asobserved earlier, it seems at best an open question whether analyticprinciples have the power to take us to realist knowledge of themathematical and modal realms.

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