Emergent Properties

First published Mon Aug 10, 2020

The world appears to contain diverse kinds of objects andsystems—planets, tornadoes, trees, ant colonies, and humanpersons, to name but a few—characterized by distinctive featuresand behaviors. This casual impression is deepened by the success ofthe special sciences, with their distinctive taxonomies and lawscharacterizing astronomical, meteorological, chemical, botanical,biological, and psychological processes, among others. Butthere’s a twist, for part of the success of the special sciencesreflects an effective consensus that the features of the composedentities they treat do not “float free” of features andconfigurations of their components, but are rather in some way(s)dependent on them.

Consider, for example, a tornado. At any moment, a tornado depends forits existence on dust and debris, and ultimately on whatevermicro-entities compose it; and its properties and behaviors likewisedepend, one way or another, on the properties and interactingbehaviors of its fundamental components. Yet the tornado’sidentity does not depend on any specific composing micro-entity orconfiguration, and its features and behaviors appear to differ in kindfrom those of its most basic constituents, as is reflected in the factthat one can have a rather good understanding of how tornadoes workwhile being entirely ignorant of particle physics. The pointgeneralizes to more complex and longer-lived entities, includingplants and animals, economies and ecologies, and myriad otherindividuals and systems studied in the special sciences: such entitiesappear to depend in various important respects on their components,while nonetheless belonging to distinctive taxonomies and exhibitingautonomous properties and behaviors, as reflected in their governingspecial science laws. (The point might be generalized yet further toinclude human artifacts which are not the object of any naturalscience, but whose conditions of individuation are tied to humanlanguage and practice. But artifacts are set aside in this entry, asthese raise distinctive issues that are discussed in the entry onmaterial constitution. Whether there are composites that are neither artifactual noramenable to scientific analysis is controversial, and if there are,they plausibly will not meet candidate autonomy conditions onemergence. But this will not be explored further here.)

The general notion ofemergence is meant to conjoin thesetwin characteristics of dependence and autonomy. It mediates betweenextreme forms ofdualism, which reject the micro-dependenceof some entities, andreductionism, which rejectsmacro-autonomy.

1. Introduction

Although debates concerning the reality or precise nature of emergenceare largely driven by contemporary scientific theorizing, the basicnotion has quite a long history stretching back at least to Aristotle(384–322 BC). On Aristotle’s view, human beings, likeother “secondary” substances, arise from a distinctivearrangement of the four material elements. While the mental powers ofhuman beings require and are necessitated by such an arrangement,these powers are distinct from, and downwardly causally efficaciouswith respect to, any non-mental powers. Furthermore, Aristotle’sform/matter compound conception of material substances is consonantwith the standard emergentist stance between substance dualism andreductionism. For detailed discussion, see Caston (1997). AmongPeripatetic philosophers, Alexander of Aphrodisias (late2nd–early 3rd century AD; inOn the Soul) and Galen(129–c.200; inOn the Elements According to Hippocratesand elsewhere) develop distinctive versions of Aristotle’s basicemergentist picture to apply to chemical compounds and othernon-living phenomena as well as living beings. (For discussion ofseveral texts, see again Caston 1997: 347–353.)

Aristotle’s philosophy of nature is re-appropriated in themedieval era, first by Persian and Arabic philosophers such asAvicenna/Ibn Sina (980–1037) and then, with the translation ofmany of Aristotle’s key texts into Latin, by philosophers in theWest, including Thomas Aquinas (1225–1274). In the Latin West,Aristotelian ideas, once introduced, become pervasive throughout theera. In particular, Aristotle’s matter/form conception ofsubstance and his concomitant rejection of atomism become axiomaticpoints of departure in theorizing about the nature of particular kindsof bodies and the distinctive kinds of processes associated with them.(On Aquinas, see Pasnau 2001 and Stump 2003, Part II; Pasnau 2011examines in detail a wide range of views of substance in thepost-Aquinas scholastic period.)

The consensus around the Aristotelian philosophy of nature wasdismantled by the Scientific Revolution, with Aristotle’sphysics being the first casualty. René Descartes advances anausterely mechanistic and reductionist conception of material bodies,and this broad outlook becomes widespread. However, Descartes arguesthat the human mind or soul is a non-material substance, and soendorses a substantial form of mind-body dualism. For those alsoaccepting the reductionist conception of the physical world, thealternatives to Descartes’ substance dualism are stark: idealism(on which matter is a mere “phenomenon” to be analyzed interms of sensations, as advocated by George Berkeley) or reductionistmaterialism, as exemplified by Julien de la Mettrie’sL’homme Machine (Man the Machine, 1747).

This menu of options is rejected in the nineteenth century by theso-called British Emergentists (with Lewes [1875] first using the term“emergence” for the philosophical position). In “Onthe Composition of Causes” (A System of Logic, 1843:Ch. 6),John Stuart Mill (see entry) argues that the behavior of living beings involves a failure ofaggregativity or linearity of influence among their elements. Heproposes an account that distinguishes “homopathic” and“heteropathic” laws and effects involving organizedphenomena, maintaining that the latter laws (governing emergentphenomena) supplement without supplanting basic physical laws of moregeneral scope.

Samuel Alexander (see entry) suggested that life was a “new quality” that emerges fromphysico-chemical processes and brings with it “special laws ofbehavior” and which must “be accepted with the‘natural piety’ of the investigator. It admits noexplanation” (1920: vol.2, 46–47). Even so, he insiststhat he is endorsing “a species of the identity doctrine”(1920: 9), which suggests that he is seeking to articulate a moreintimate relationship between levels of the natural world, perhapsvery much akin to weak emergence accounts discussed insection 3 below.

Finally, British Emergentism reaches its most developed form inC. D. Broad’s (see entry)The Mind and Its Place in Nature (1925). Broad uses anepistemological criterion for what he intends to be a metaphysicalcondition of emergent autonomy:

the characteristic properties of the whole R(A, B, C) [where R markstheir structural arrangement] cannot, even in theory, be deduced fromthe most complete knowledge of the properties of A, B, and C inisolation or in other wholes which are not of the form R(A, B, C).(1925: 61)

He adds that emergent features are “completely determined”by such lower-level features, in that

whenever you have a whole composed of these […] elements incertain proportions and relations you have something with the[compound’s] characteristic properties and […] nothinghas these properties except a whole composed in this way. (1925: 64)

Reminiscent of Mill, he distinguishes “intra-ordinal” fromemergent “trans-ordinal” laws (1925: 77–8) which,although dealing solely with complex phenomena, are “unique andultimate” (1925: 64–5). As a consequence:

On the emergent theory we have to reconcile ourselves to much lessunity in the external world and a much less intimate connexion betweenthe various sciences. At best the external world and the varioussciences that deal with it will form a kind of hierarchy. (1925:78)

The popularity of the emergentist vision waned beginning shortly afterBroad’s writing, with important scientific developments erodingthe boundaries between adjacent “levels” (most notablyquantum chemistry and molecular biochemistry—see McLaughlin 1992for discussion). However, attention to emergence was substantiallyreinvigorated starting in the 1970s with the discovery and creation ofnon-linear complex systems of both natural and artifactual varieties.There is now a large recent literature developing accounts ofemergence motivated by such systems. It has gradually broadened toencompass mental phenomena that seem to resist physicalist treatment(e.g., consciousness and free will) and some have even begun tore-think received wisdom concerning the relationship of chemistry andbiology to lower-level sciences. These debates are of considerableimportance for our understanding of the natural world and of our ownplace within it. As the bibliography attests, the questions at issuehere engage philosophers and scientists alike. This should beunsurprisingly, for these questions are neither simply empirical norwhollya priori in character, but are rather such thatplausible (if not uncontroversial) answers require consideration ofand support for metaphysical interpretations of the structure ofnatural reality in light of our best empirical theories.

Affirming that thereare emergent phenomena that are bothdependent on and yet autonomous with respect to underlying physicalstructures leaves a great deal open, however, as both of thecharacteristics have been explicated in diverse ways (see Van Gulick2001, Gillett 2002, Wilson 2015, and Humphreys 2016 for discussionshighlighting this diversity). Accounts of the dependence at issue mayappeal to relations of composition, supervenience, realization,grounding, or causation. Accounts of the autonomy at issue may appealto fundamental or non-fundamental novelty of powers, properties,forces, laws, or effects; irreducibility; non-aggregativity; ornon-linearity.

Though diverse, accounts of ontological emergence can be usefullygrouped by a basic division between those that are and are notcompatible withphysicalism, understood as the thesis thatall natural phenomena are wholly constituted and completelymetaphysically determined by fundamental physical phenomena. Thisthesis is standardly understood to entail “the causal closure ofthe physical”, according to which (roughly) anyfundamental-level physical effect has a purely fundamental physicalcause. “Strong” emergence accounts are inconsistent withphysicalism and causal closure (as just elucidated) while weakemergence accounts are consistent with it.

The general division into weak and strong varieties of ontologicalemergence frames much of the discussion to follow:

Section 2 (“Ontological emergence: features”) surveysthe main options for understanding the primary characteristics ofdependence and autonomy, noting whether they are more commonlyassociated with weak emergence, strong emergence, or both.
Section 3 (“Weak emergence”) explains the primarymotivations for and challenges to weak emergence as a general approachand presents a range of possible accounts.
Section 4 (“Strong emergence”) explains the primarymotivations for and challenges to strong emergence as a generalapproach and presents a range of possible accounts.
Section 5 (“Weak vs strong emergence: contestedphenomena”) canvasses phenomena recently proposed as potentialcases of (the more controversial) strong emergence and briefly notesarguments for and against their having that status.

2. Ontological Emergence: Features

This section considers the core features of dependence and autonomy inturn, noting the variety of ways theorists of emergence haveelucidated them.

2.1 Dependence

Emergents depend on micro-configurations. That theobjects orsystems manifesting emergent features and patterns are exhaustivelycomposed by lower-level physical entities is assumed by almost alltheorists (but seesection 4.3). But there are at least three different ways one might suppose thefeatures and patterns themselves to depend on the features andpatterns exhibited by those parts.

2.1.1 Dependence: modal (supervenience)

Accounts of emergence of both strong and weak varieties typicallysuppose that emergents modally depend on their physical bases, suchthat it is necessary that if an emergent occurs, some or otherphysical basis occurs, and it is further necessary that if that basisoccurs the emergent occurs. The holding of both directions ofnecessary correlation corresponds to what is called“strong” supervenience (Kim 1984: 165):

Astrongly supervenes on B just in case, necessarily, foreach x and each property F in A, if x has F, then there is a propertyG in B such that x has G, andnecessarily, if any y has G, ithas F.

Here A and B are families of properties (the supervening andsupervenience base properties, respectively).

Emergence may differ with respect to the strength of these two modalcorrelations. Accounts of weak emergence typically specify thenecessity as metaphysical necessity (absolute or unconditionalnecessity), indicating that emergent phenomena are (in a somewhat hazysense) “nothing over and above” their subvening basephenomena. Accounts of strong emergence specify it as (merely)nomological necessity (conditional on the holding of relevant laws ofnature), indicating a greater ontological distinctness betweenemergent features and their bases. Indeed, it has sometimes beensuggested that the difference between strong and weak varieties ofemergence is best understood in terms of the distinction betweenmerely nomological and metaphysical supervenience (Chalmers 1996,Noordhof 2010).

2.1.2 Dependence: functionally realized

Philosophy of mind in the past half-century has been dominated by thethesis that (many) mental concepts are functional concepts, specifyingmental states (or properties or events) in terms of the functional orcausal roles they play in response to incoming stimuli, in relation toeach other, and in generating goal-directed behavior (see, e.g.,Putnam 1967, Fodor 1974, and Shoemaker 1984). Functionalists who arealso physicalists maintain that mental states are “functionallyrealized”, in that their defining roles are implemented, on anygiven occasion, by lower-level physical processes. As a schematicexample, the mental state of being a certain kind of physical pain isa state caused by certain kinds of injury to one’s body, andwhich causes certain kinds of behaviors in response (certain vocalexpressions, treating or getting help for the injury, etc.), and(supposes the physicalist), this functional/causal role is alwaysplayed by some lower-level physical process.

One account of (weak) emergent dependence is just this sort offunctional realization. A concomitant of such a functionalist approachis that any token power (i.e., particular instance of it) associatedwith a weak emergent feature will be identical to a token power of itsrealizer (Wilson 1999, 2015). This will render emergence compatiblewith the physicalist thesis of physical causal closure. However, ifproperties have non-causal quiddities—primitively individuatingaspects that are the property equivalent of individualhaecceities—additional to their causal powers, then tokenidentity of powers will not necessarily guarantee that a functionallyrealized emergent property is dependent on its lower-level base in away compatible with physicalism (Melnyk 2006). (It has become commonin recent metaphysics to invoke a primitive notion of“grounding” relating the more fundamental to the lessfundamental, so the reader may be surprised not to see it listed as adistinct option here. The reason is two-fold: it is not often used bytheorists of emergence, and it is a generalized notion thatencompasses the present, more specific notion of functionalrealization. For discussion of the relative merits of deploying morespecific notions or a more general unifying notion in discussions offorms of ontological dependence, see the exchange between Wilson 2014and Schaffer 2016.)

2.1.3 Dependence: nomological or causal

Accounts of strong emergence commonly characterize dependence as(merely) nomological or causal. Such external relations are consistentwith strong emergents being “over and above” the physicalentities upon which they depend in a straightforward way. It alsoleads to a rejection of physical causal closure, often through theposit of fundamentally novel “compositional” powers orforces (as described by, but not endorsed in McLaughlin 1992). Onemight take strong emergents to be diachronically caused by base-levelevents (O’Connor & Wong 2005) or maintain that they aresynchronically nomologically dependent on their bases, with newfundamental interactions, powers, or laws appearing solely at theoperative level of complexity (Broad 1925).

2.2 Autonomy

Discussion of stronger and weaker varieties of emergent dependenceinevitably hints at differences in the ways one might specify thecorrelate thesis that emergents are to some extent autonomous fromtheir physical bases. The commonality to the genus of autonomy isdistinctness: emergents are not identical to theirbases—at least at the level of types, and perhaps also at thelevel of tokens. What follows are three central ways of understandingthe source of such distinctness.

2.2.1 Autonomy: non-aggregativity

Emergent autonomy might be characterized in terms of a specificelucidation of the idea of compositional or structural“non-aggregativity”. Wimsatt (1994), an early proponent ofnon-aggregativity as key to emergence, defines aggregativity in termsof the “associativity, commutativity, inter-substitutivity,linearity, and invariance under decomposition and reaggregation”of a system’s parts (1994: 237), with varieties ofnon-aggregativity simply being the failure of such conditions. Moredirect and positive characterizations of non-aggregativity appeal todistinctive, self-organizing group-level behaviors, such as antcolonies and flocking birds, whereby “interaction among theparts generates properties which none of the individual componentspossess” (Mitchell 2012: 179). (Other developments of thisapproach are found in Kauffman 1993 & 1995, Thompson & Varela2001, Camazine et al. 2001, and Thompson 2007.)

2.2.2 Autonomy: multiple realizability

A second way one might characterize the autonomy of emergent entitiesis through realization by base properties that does not depend on anyspecific realization. Physical realization secures a clear form ofdependence on base properties (section 2.1.2). But realized functional properties and processes are (or appear tobe) consistent with a variety of possible realizer properties (e.g.,having a headache, even of a highly specific type, is consistent withdifferent precise neural states, and more general mental state typesmay be realized more widely still, across biological species andperhaps in alien or artificial minds). They are multiply realizable,dependent on there being some physical property or process or otherwithin a range, without depending on anyspecific suchfeature. There are other varieties of realization beyond thefunctional that might be apt candidates for understanding particularcases of emergent autonomy. For example, emergent properties might bedeterminables which are determined by different lower-leveldeterminates (Macdonald & Macdonald 1986, 1995 and Yablo 1992);types and/or tokens which are proper parts of lower-level types ortokens (Clapp 2001 and Shoemaker 2000 [2001]); or having strictlyfewer degrees of freedom than do their bases (Wilson 2010). Theseoptions, which need not be mutually exclusive, are discussed furtherinsection 3.2.

Multiple realizability, or something akin to it, is expressed incontemporary physical sciences by the notions of universalizability(where diverse systems behave in similar ways), stability underperturbation, and eliminations in micro-physical degrees of freedom,and many writers point to such phenomena as instances of emergence.See Batterman (2000 and 2001) for detailed examination of thescientific application of these notions.

2.2.3 Autonomy: distinctive efficacy

Almost all accounts of emergence suppose that emergents are not justdistinct from, but also distinctively efficacious as compared to,their bases. Weak emergentists typically deny that emergents have anyfundamentally new powers, on grounds that such new powers would be,either directly or indirectly, powers to produce certain basicphysical effects, violating the causal closure of the fundamentalphysical realm. They maintain, however, that distinctive efficacy doesnot require having a new power. Distinctive efficacy might beunderstood in terms of distinctive counterfactual patterns over time,yoked to a counterfactual account of causation (LePore & Loewer1987 and 1989); difference-making or “proportionality”considerations in accounting for macroscopic as against microscopiceffects (Yablo 1992); or more generally through the proposal thatspecial science laws track comparatively abstract levels of causalgrain (Antony & Levine 1997, Wilson 2010).

Strong emergentists, by contrast, standardly take emergents tointroduce fundamentally novel causal powers—powers that theirlower-level physical bases do not have. These are often taken to bepowers of the high-level features themselves, and are directed“downwardly” at the structures from which they emerge (aswell as “horizontally” in contributing to emergentfeatures of the system at subsequent times). But some propose thathigh-level structural features induce novel powers incomponent entities; others suggest that systemic“transformations” occur in which parts either lose theiridentity when caught up in emergent wholes or have their behaviortransformed in virtue of such embeddedness. Details concerning theseproposals are provided insection 4.2 below.

3. Weak Emergence

Weak emergence affirms the reality of entities and features posited inthe special sciences, while also affirming physicalism, the thesisthat all natural phenomena are wholly constituted and completelymetaphysically determined by fundamental physical phenomena, entailingthat any fundamental-level physical effect has a purely fundamentalphysical cause.

Special sciences describe non-ubiquitous, structured phenomena (e.g.,plate tectonics, molecular interactions, cellular repair, andorganismic development) and successfully predict their behaviorthrough higher-level laws. Weak emergentists take the existence ofsuch stable and distinctive phenomena, amenable to high-level but notlow-level explanation, as reason to accept the taxonomic categories ofthe special sciences into our ontology of the natural world, no lessreal than the categories of a final, completed physics. On this view,thereare molecules, cells, organisms, and minded creatures,and they do not reduce to—are not identical to—complexcombinations of basic physical entities or features. Correspondingly,explanations adverting to special science laws are to some extentautonomous from the explanations adverting to laws of lower-levelphysical theories.

3.1 Challenges to its viability

This section introduces two commonly pressed challenges to thein-principle viability of weak emergence. The accounts of weakemergence in the subsequent section are partly motivated as ways ofmeeting one or both of the challenges.

3.1.1 From parsimony

The simplest challenge to weak emergence is from parsimony, and it canbe pressed from the side of reductionism or anti-realism (e.g., Heil2003 and Ney 2010) or from the opposing side of strong emergentism(e.g., O’Connor & Churchill 2010a,b). The weak emergentistgrants that the ontology and dynamical laws of a completed, truephysics metaphysically determine all fundamental physical facts,and that the latter metaphysically determine allnon-fundamental facts or associated truths about the world. In thatcase, why add to the ontology of the world? Why not parsimoniouslyrest with the lower-level physical ontology, and offer a deflationaryaccount of truths that appear to refer to “higher-level”phenomena? One might, e.g., construe the phenomena as coarse-grainedpatterns running through the world of fundamental physical phenomena,and regard truths invoking concepts such ascell,metabolism,cat, anddesire as referring(perhaps schematically) to distinctive types of arrangements ofunderlying physical entities and their qualities. This stance cangrant that in practice we cannot dispense with special scientificclaims. (The associated coarse-grained patterns are in some sense thestarting point of empirical inquiry for creatures like us, and evenwere we to arrive at a true final physical theory, our cognitivelimitations would make it impossible for us to track the evolution ofthe world in terms directly corresponding to such a fine-grainedreality.) But (says the critic) such broadly pragmatic considerationsshould not guide our views concerning the world’s ontology;blocking such deflationary moves requires a stronger form of autonomythan physicalism allows.

3.1.2 From overdetermination (or “exclusion”)

Weak emergence accepts the following five premises:

Supervenient Dependence. Emergent features (properties,events, or states) synchronically depend on their base features inthat, the occurrence of an emergent feature at a time requires and isnomologically necessitated by the occurrence of a base feature at thattime.
Reality. Emergent features are real.
Efficacy. Emergent features are causallyefficacious.
Distinctness. Emergent features are distinct from theirbase features.
Physical Causal Closure. Every lower-level physicaleffect has a purely lower-level physical cause.

Jaegwon Kim (in, e.g., his 1993 and 1998) argues that these premisesentail an unacceptable conclusion:

Overdetermination. Emergent effects are generallycausally overdetermined by distinct individually sufficient synchroniccauses (akin in every case to otherwise unusual and perhaps merelypossible cases as when two rocks are thrown independently and strike atarget at the same time).

The present presentation of Kim’s argument follows Wilson 2015,with some modifications (see also the discussion in Sturgeon 1998).Emergent effects might be same-level, downward, or both(Efficacy is neutral). Suppose, first, that emergentEcauses emergentE* (same level), whileE supervenientlydepends onP andE* superveniently depends onP* (Supervenient Dependence).P* has a purelylower-level physical cause (Physical CausalClosure)—plausibly,P. IfP causesP*,andP* necessitatesE*, then it is plausible thatP causesE*, by causingP*. So bothP andE causeE*, and given thatP andE areboth real and distinct (Reality,Distinctness),E* is causally overdetermined.

Second, suppose thatE instead causes some lower-level basefeatureP*.P* also has a purely lower-level cause(Physical Causal Closure)—again, plausiblyP. SobothP andE causeP*, and given thatPandE are both real and distinct (Reality,Distinctness),P* is causally overdetermined.

Thus, whether we conceive emergent causation as same-level ordownward, the weak emergentist’s commitments entailoverdetermination (or as it is sometimes put, holding fixednon-overdetermination, emergent causation iscausallyexcluded by the ubiquity of fundamental physical causes). Findingsuch systematic overdetermination to be implausible, Kim concludesthat we should rejectDistinctness and embracereductionism.

Rejecting other premises leads to alternative stances. EliminativistsdenyRealism, epiphenomenalists denyEfficacy, andsubstance dualists and some strong emergentists denySupervenientDependence. Strong emergentists commonly denyPhysical CausalClosure, maintaining that emergent features are non-redundantcausal contributors alongside lower-level physical factors to somelower-level physical effects. Weak emergentists instead point to theintimate relation they see between emergents and their bases in ordereither to show a way to resist the argument’s overdeterminationconclusion or to make plausible that emergent and basal physicalcauses arenon-competing in a way that rendersoverdetermination harmless.

3.2 Contemporary accounts

In this section, three broad accounts of weak emergence are introducedand proposed variations noted.

3.2.1 Token identity/type non-identity

One weak emergentist approach locates the distinctness and distinctiveefficacy of emergents in feature or processtypes, whilemaintaining that every token feature or process is identical with atoken fundamental physical entity. This view in a sense “splitsthe difference” with the reductionist, and so is the mostminimalist of the weak emergent positions. One advantage of the viewis that it provides a basis for resisting Kim’s argument forcausal overdetermination. Causation is a token-token relation. Iftoken higher- and lower-level entities are identical, then there isbut one token cause of a given effect, which seems to dismantle thechallenge. At the same time, the type-distinctness of weakly emergentand base features provides a basis for claiming distinctive efficacythrough “causal relevance” considerations. We may querythe cause of a tokeneffect through the prism of distincteffect types, and which type we select will determine whichcause type is relevant to its explanation.

Proponents of such accounts typically offer some specific account ofthe relation at issue, aimed at rendering it plausible that thefeatures at issue might be type-but-not-token distinct. Cynthia andGraham Macdonald (1986, 1995) suggest that higher-level features aredeterminables of lower-level physical features and determinable typesare reducible to disjunctions of determinate types. Given thiscombination of views, higher-level determinable features aretype-distinct from associated lower-level determinate features (sincea disjunctive type is not identical to any one disjunct type), butevery instance of a higher-level determinable feature istoken-identical with some lower-level determinate feature (sinceinstances of disjunctive types are token-identical with instances ofsome or other disjunct type). Some critics object that higher-levelfeatures are not appropriately modeled as determinables of lower-levelphysical determinates (Ehring 1996), or that determinables cannot bereductively analyzed in terms of determinates (Wilson 2012).

Robb (1997) and Ehring (1996) offer token identity accounts on which features of thingsare “tropes” (particularized properties, such as thealleged particular whiteness of a given sheet of paper) and propertiesper se are sets or collections of tropes (seeentry on tropes). We may suppose that every higher-level trope is identical with alower-level physical trope, notwithstanding that the one trope is amember (or part) of different property-identifying sets of tropes, andhence of different types. Here again, the suggestion is that such aview avoids overdetermination while preserving a role for thedistinctive efficacy/causal relevance of higher-level features.

3.2.2 Realization

Perhaps the most common approach to weak emergence is one appealing toone or other account of “realization”. (Recall the pointmade insection 2.1.2 that realization falls under the more general concept of groundingthat is deployed more frequently in some other contexts.) The firstaccounts of realization were in functionalist terms, according towhich a realized feature is characterized by a distinctive causal orfunctional role, which role is implemented, on any given occasion, bysome or other lower-level physical feature. Hence, Putnam (1967)suggests that, just as a software program might be implemented onmultiple hardware platforms, so mental states might be associated withcausal-functional roles which are played by multiple neurological, andultimately fundamental physical, states. Such an approach accommodatesthe ontological autonomy of weak emergents to the extent that afunctionally associated type is not identified with any one of itsrealizer types. It would also appear to provide a basis for blockingconcerns about causal overdetermination by analogy again to softwareprograms, whose powers areinherited from their lower-levelrealizers. Given such an inheritance thesis for higher-level powers,even if there is a sense in which both realized and realizing featurescan cause a given effect,only one power is exercised, nottwo—in which case, it is maintained, the overdetermination isbenign. (This functionalist perspective on special science entitiesand features was influentially developed by Fodor 1974.)

Other approaches to realization appeal to other specific relations. Asnoted above, one might endorse an account of weak emergence in termsof determination through the assumption that instances ofdeterminables are token-identical to instances of their realizingdeterminates. But one might adopt this general approach whilerejecting the assumption of token identity, either on grounds thatdeterminable tokens as well as types have less specific essences thanassociated determinate tokens and types (Yablo 1992) or on groundsthat determinable tokens as well as types are associated with fewerpowers than associated determinate tokens and types (Wilson 1999,2015). Yet another approach to realization is based in the part-wholerelation, with higher-level types and tokens taken to be proper partsof lower-level types/tokens (Shoemaker 2000 [2001], Clapp 2001).Common to these particular strategies is the idea that token and typepowers of a realized feature are a (non-empty) proper subset of thetoken or type powers of the lower-level physical feature upon which itdepends (Wilson 1999, 2015). Causal autonomy can result from ahigher-level feature’s having fewer, not more, powers than thefeature upon which it depends because the latter encodes distinctivedifference-making (e.g., if one’s neurological state had beenslightly different, one would still have been thirsty and still havereached for the glass).

Finally, it has recently been proposed that a distinctive form ofphysical realization comes through implementingmechanisms:enduring structures of organized components that realize higher-levelrole functions, endowing higher level entities with novel,non-aggregative behaviors and properties (see Machamer et al. 2000,Craver 2007, andentry on mechanisms in science). Craver (2007) argues that in neuroscience and psychology inparticular, we should think of minded biological agents as constitutedby a hierarchy of mechanisms (or networks of such mechanisms). Whilehe avoids use of the term “emergence”, which he associateswith strong emergence (2007: 16), he is plausibly taken to advance adistinctive form of weak emergence. He defends the reality of suchhigh-level, fully-realized powers on the grounds that they satisfy theconditions for causal relevance on the (manipulationist) account ofcausal explanation that he favors (2007: 216–220). Haug (2010)develops a mechanistic account of realization incorporating adistinction between what he calls “constitutive” and“integrative” mechanisms, associated with two distinctroles mechanisms play in the realization of special scienceproperties. He argues that his mechanistic account contrasts favorablywith determinable/determinate approaches (and more generally,“proper subset of powers” approaches) to realization, andthat it provides a principled basis for claims of multiplerealizability. (Gillett 2016 also emphasizes realizing mechanismsassociated with novel powers in his account of emergence. But as herejects physical causal closure, his account will be discussed underthe rubric ofstrong emergence insection 4.)

3.2.3 Explanatory “incompressibility”

The appeal to unpredictability or other epistemic criteria plays asignificant role in accounts of weak emergence (going all the way backto Broad’s [1925] highly influential discussion), even thoughthe intended contrast with “reductionism” is ontological.It is near the surface in the non-aggregativity account of emergentautonomy summarized insection 2.2.1 above, in which the non-linearity of macro-system dynamics (resultingin practical unpredictability) is taken to be a hallmark of (weak)emergence. Bedau (1997, 2010) proposes that we build this distinctivediscovery of modern complex systems research into our theoreticalelucidation of emergence. Specifically, a feature of or process withina macro-system is weakly emergent just in case it is derivable fromthe prior micro-facts leading up to it, but only in an informationallyincompressible way: describing its macro-state at prior times byaggregating all of its underlying micro-states at those times anditerating their micro-dynamics. When a system’s macro-evolutionis explanatorily incompressible in this way, it is notdynamicallycharacterizable in fundamental physical terms: its distinctiveevolutionary patterns are not foreshadowed in any way in themicro-evolutionary patterns.

It is possible to see special cases, giving rise to distinct speciesof weak emergence, within the broad framework of non-linear dynamics.Rueger (2000) and McGivern and Rueger (2010) suggest that a form ofdiachronic (or evolutionary) emergence occurs when there is a sharpchange in the behavior of a single system—behavior which is“qualitatively novel” with respect to its earlierbehavior—as a result of a comparatively small perturbation inone of its underlying control parameters. There are well-documentedcases of such transitional changes in a system’s behavior.

4. Strong Emergence

Strong emergentists maintain that at leastsome higher-levelphenomena exhibit a weaker dependence/stronger autonomy than weakemergence permits. This often takes the form of rejecting physicalrealization, affirmingfundamental higher-level causalpowers, or both.

Perhaps the most commonly cited phenomena offered as requiring strongemergentist treatment have to do with the nature and capacities of theconscious mind in relation to its neural substrate. Other non-mental,scientific phenomena also have been advanced as possibly or plausiblyrequiring treatment in strong emergentist terms. Such claims arecanvassed insection 5.

4.1 Challenges to its viability

This section introduces three commonly pressed challenges to thein-principle viability of strong emergence. Possible replies are notedin connection with certain accounts of strong emergence introduced inthe subsequent section.

4.1.1 Incoherence or inexplicability

An initial worry about strong emergence is that there is a tension inthe very idea of a feature that is both dependent andfundamental—a worry exacerbated by recent accounts offundamentality according to which what it is to be fundamental isprecisely to be independent (see Bennett 2017 andentry on fundamentality).

This worry might be resolved by distinguishing two senses of“fundamental”: first, a sense applying to an ingredient ofphysical reality that is ubiquitous (or “basic” in abuilding-block sense), and so not even dependent on any arrangementsof other entities; and second, a sense applying to an ingredient ofreality that is not (entirely) constituted by or otherwise internally(as opposed to external-causally) related to the structuredarrangement of some other same-category entities. We might then usethe term “basic” for the first sense and reserve“fundamental” for the second sense, which aligns thelatter term with the way it is used in contemporary discussions of“grounding” or metaphysical dependence in metaphysics. Sounderstood, there is no tension in the notion of an entity or featurethat is fundamental but non-basic (O’Connor 2018).

Granting that there is no incoherence in the idea of a non-basic,fundamental entity or feature, one might be concerned that such anentity or feature would introduce an inexplicable (since fundamental)addition to reality at an arbitrary juncture. Avoiding suchinexplicability might give reason to prefer a panpsychistaccommodation of the irreducibility of consciousness to physicalproperties, as it (unlike strong emergence) posits proto-consciousqualitative character into thebasic structure of the world(Nagel 1979 and Strawson 2006).

4.1.2 Anti-naturalism or evidential paucity

A second initial worry with strong emergence is that it isinconsistent with a “naturalist” point of view, insofar as(on most accounts) strongly emergent properties are associated withfundamentally novel powers or laws that apparently would interferewith more basic physical laws or processes. However, basic dynamicallaws in contemporary physics have an open-ended character(Schrödinger’s equation, Hamiltonians or Lagrangians moregenerally), taking forces or energies as input. The notion of astrongly emergent force or energy is no more problematic than that ofthe standard physical forces or energies that physicists take to beinput into the operative laws (McLaughlin 1992).

The real problem here, if there is one, is not inconsistency withphysics, but rather that there is at present a lack of clear empiricalevidence for strong emergence. If there were strongly emergent causalpowers, forces, or laws, we might expect to see, in candidateemergentist contexts, evidence for a hitherto unrecognizedconfigurational interaction, much as occurred with the weak nuclearinteraction. But, McLaughlin avers, “there is not a scintilla ofevidence” in support of there being such fundamental novelty(1992: 91). For strong emergentist replies to this contention, seesection 4.2.2 andsection 5.

4.1.3 Collapse

A final challenge to the viability of strong emergence is that suchaccounts are vacuous, in that any purportedly fundamentally novelpower possessed by an emergent is, on any of a number of plausibleaccounts of powers, “already” possessed by its lower-levelbase goings-on.

On one version of this “collapse” objection, certain waysof individuating lower-level physical features entail that suchfeatures will have dispositions to produce any purportedly stronglyemergent features, undermining the supposed metaphysical novelty ofthe emergent features in favor of an enriched understanding of thebase features (O’Connor 1994: 98–9 and 2000: Ch.6,introduces this objection; see Howell 2009 and Taylor 2015 fordevelopment). On another version of the objection, certain ways ofassigning powers to features entail that lower-level physical featureswill inherit any powers had by purportedly strongly emergent features(Kim 1998). For strong emergentist replies to the“collapse” challenge, seesection 4.2.2.

4.2 Contemporary accounts

I now discuss a representative range of contemporary accounts of thisform of emergence.

4.2.1 Formal characterizations

A metaphysically minimalist characterization of strong emergence issupervenience-based. In contrast to weakly emergent or physicallyreducible features, which are taken to supervene with metaphysicalnecessity on their physical dependence base, strongly emergentfeatures are taken to supervene with merely nomological necessity (vanCleve 1990, Chalmers 2006, Noordhof 2010).

This approach assumes that the conceptual distinction betweennomological and metaphysical necessity corresponds to a substantialone, such that fundamental causal laws are metaphysically contingentand what supervenes of metaphysical necessity on the fundamentalphysical domain is necessarily physical. Both these assumptions havebeen challenged. Moral anti-naturalists such as G.E. Moore maintainthat fundamental moral features supervene of metaphysical necessity onnatural features, and on neo-Aristotelian essentialist ontologies,strongly emergent features, were they to exist, would supervene withmetaphysical necessity on physical features. See, however, Howell(2009) and Noordhof (2010) for strategies aimed at defendingsupervenience-based approaches to strong emergence.

One variant of this approach is epiphenomenalist (Jackson 1982, Kim2005, and Chalmers 1996 all express sympathy for this view). Itpresumes that the most plausible candidates for strong emergence arethe qualitative features of conscious experience—e.g., the waythat the redness of a rose looks to a particular subject on a givenoccasion. On this approach, such “qualia” (qualitativefeatures) are caused or otherwise nomologically necessitated by theneural processes underlying human experience, but they are themselvesincapable of producing any effects, including on any mental statesgenerated by the purely physical aspect of the perceptual process.

Epiphenomenal strongly emergent features are consistent withphysical causal closure. For this reason, the account is lessvulnerable than standard accounts to the objection that it entailspresently unsupported empirical predictions about brain processes. (Itis still incompatible with physicalism, since strongly emergent qualiaare not physically realized, and so are wholly additional to tokenlower-level physical phenomena.) A special challenge for the view isthat qualia would seem (paradoxically) explanatorily irrelevant to asubject’s beliefs concerning their existence (see Chalmers 1996:Ch.5, for possible responses).

On a second formalist explication, strongly emergent phenomena arethose which are both fundamental and dependent, and where the notionsof fundamentality (whose converse is the “grounded”) anddependence are each taken to be primitive (Barnes 2012). Such aprimitivist account clearly blocks ontological reduction: if somegoings-on are fundamentally novel vis-à-vis the goings-on uponwhich they depend, then the former are clearly not identical to thelatter. A potential advantage of the approach is that it might capturethe common denominator to all other, more distinctive accounts ofstrong emergence. A corresponding shortcoming is that its high levelof abstraction leaves it without the resources of more specificaccounts for addressing the general challenges facing strong emergencenoted above (Paolini Paoletti 2018 and Pearson 2018).

4.2.2 Fundamental powers or laws

Another account of strongly emergent autonomy posits fundamentally newpowers, forces, or laws directly associated with the emergent propertyof a complex object or system (O’Connor 1994, Silberstein &McGeever 1999, Wilson 2002, and O’Connor & Wong 2005).“Fundamental” here signifies that the power is not theresultant of any generalized kind of summation on the powers of theobject’s constituents relative to the relational structure inwhich they are embedded. One variation on this basic account concernsthe way emergentsdepend on their bases. One common variantholds that emergent features are synchronically nomologicallynecessitated by the base-level structure, where the necessitation,while law-governed, is not via causal production (Broad 1925,Silberstein & McGeever 1999, and Wilson 1999 and 2002). Analternative (naturally suited to a neo-Aristotelian causal powersmetaphysics) maintains that emergent features are diachronicallycaused by a plurality of micro-properties acting jointly (perhapstogether with other emergent features), under the triggering conditionof the micro-properties’ being appropriately structurallyorganized (O’Connor & Wong 2005). This alternative can allowthat, even granting that causal necessity is a species of metaphysicalnecessity, the supervenience of strongly emergent features on basephenomena will fail in (at least possible) scenarios in which thecausal dependence between either base and emergent features oremergent features and their effects is non-deterministic. That said,whether supervenience does in fact fail for any actual cases of strongemergence is an empirical question (on which, seesection 5.2).

The fundamental powers account of strong emergence is consistent witha range of replies to the challenges noted insection 4.1. The first challenge asserts that strongly emergent features are“inexplicable”, insofar as they appear only in certainorganized structures and could not be predicted in principle throughthe laws of basic physics applied to emergent-antecedent conditions.One reply is simply to observe that in the history of physics itselfthere is precedent for adding new fundamental laws to explain whatpreviously recognized laws could not (e.g., electromagnetism and theweak nuclear force—see Wilson 2002). When determining thefundamenta of the natural world, we must go where the evidence leads.An alternative reply in line with the second variant of the previousparagraph denies the charge of inexplicability by supposing thatemergent features are the product of micro-structural dispositionsthat, unlike those which are more or less continuously manifested,have structural triggering conditions. A Laplacian observer of theunfolding universe prior to the initial onset of such emergentfeatures would have no inkling of there being such latent dispositionstowards collective emergent effects. But they are as fundamental asthe more generally manifested dispositions associated with suchproperties as mass and charge (O’Connor & Wong 2005). And itmight be contended that while it is rational to presume that thenatural world is a causallyunified totality, we cannotpresume that it is causallyuniform (O’Connor 2000:Ch.6).

The collapse challenge asserts that any purportedly strongly emergentpowers would “collapse” into—that is, end up beingassociated with—their dependence bases. One might seek to blockcollapse by distinguishing between collective dispositions forproducing certain powers and the powers themselves, or between directand indirect having of powers; or by characterizing emergent powers asrelative to specified sets of fundamental interactions (seeO’Connor 1994: 98–99, and 2000, Ch.6, Wilson 2002, andBaysan & Wilson 2017). Two other reasons for resisting attempts tointerpret (possible) discontinuous microphysical behavior in terms ofmicro-physical powers alone are epistemological. One might contendthat we should seek to provide as unified an explanation as possibleof such phenomena, and that an explanation in micro-physical termsalone would be highly complex and disunified compared to one thatposits a family of emergent macroscopic determinables fitted tostructures of organized complexity. Second, we can describe certainpossible situations involving causal indeterminism that would bereadily understandable in strong emergentist terms, whereas the onlymicrophysical explanation possible would require an objectionableaction at a temporal distance. (See O’Connor & Wong 2005 fordevelopment of these replies.)

Finally, possible responses to the final, evidential challenge tostrong emergence are best considered insection 5, where a range of candidate emergent phenomena are introduced.

4.2.3 Novelcomponent powers in configured wholes

Notwithstanding the replies just above to the collapse challenge, arecent trend in theorizing about strong emergence has been to embracecollapse, arguing that emergence is better understood as theintroduction of novel powers had bycomponents when embeddedin configured wholes. This section introduces four ways this accounthas been developed.

In a series of articles culminating in a 2016 book, Carl Gillettadvances a distinctive account of strong emergence rooted in ahierarchy-of-mechanism picture of complex systems that, he maintains,is strongly supported by a range of sciences. Gillett invokesconsiderable conceptual machinery in developing his view; makingsubstitutions in linked definitions, we arrive at the followingcompact statement:

A property is strongly emergent just in case it is aproperty of a composed individual that is realized and that (inaddition to having same-level effects) non-productively determines theindividual’s parts to have powers that they would not have givenonly the laws/principles of composition manifested in simplercollectives.

There are two distinctive features of Gillett’s account. Asnoted, the account ascribes fundamentally novel properties not to thebearer of the emergent property, but to the bearer’s components.Second, a realized structural property of the whole is nonethelesssaid to be the emergent property in virtue of its“nonproductively determining” the components’ comingto have those novel powers. It is tempting to think of this last asalso ascribing a novel (albeit non-productive) power to the emergentproperty, although Gillett does not describe it in these terms. Notethat Gillett’s account does not squarely fit the strongemergence classification given in this entry, insofar as he takesstrongly emergent properties to be “realized by”lower-level properties, when standing in a complex relation;nonetheless, the account properly belongs to the present strongemergent classification to the extent that it is inconsistent with thecausal closure of the microphysical realm.

As regards the general charge that strong emergence is inexplicable,Gillett maintains that this account provides an intelligible,scientifically informed basis for making sense of strong emergence. Asregards the charge of there being no evidence of strong emergence,Gillett suggests that while its reality has not been establishedempirically, it is seriously proposed by a number of contemporarytheorists of complex systems (seesection 5.2 andsection 5.3 below).

David Yates (2016, forthcoming) has proposed an account with somesimilarities to Gillett’s. He discusses the way that the bentgeometry of a water molecule determines its dipole moment, whichlatter feature confers a range of causal powers on the molecule, suchas its disposition to align in an electric field and its being liquidat room temperature (2016: 822–225). He argues that thisgeometrical property, while fully realized by the spatial relationsbetween the molecule’s atoms, confers anew conditionalpower on the molecule that in tandem with the causal powers inheritedfrom the molecule’s basic constituents enables the molecule toproduce its characteristic effects. Key to Yates’s proposal isthe suggestion that higher-level features may be“qualitatively”, as opposed to functionally, realized:while a functionally realized property is characterized in terms ofderivative causal powers, qualitatively realized properties arenon-causally (e.g., spatiotemporally) characterized, making room forthem to be causally fundamental. As he puts it:

[Q]ualitatively realized properties are defined bynon-causalspecifications, and their realization is not a matter of bestowing theright causal powers. Properties such as molecular geometry arecausally fundamental […] because their bearers have certaincausal powers in virtue of meeting their defining specifications, butnot in virtue of the realizer properties in virtue of whichthey meet those specifications on a given occasion. (2016: 812)

Yates contends that these examplesdemonstrate the reality ofhis distinctive variety of strong emergence. A skeptic might pressthat the effects Yates cites as pointing to a distinctive kind ofhigher-level causal power are themselves all higher-level. Assumingthat all macroscopic properties are microphysically realized, if onewere able to take a wide-angle view of the evolving process in purelymicro-physical terms throughout (including in characterizing thetargeted token effects), it’s not clear that reference toanything other than the features of and basic relations amongmicrophysical entities is required for explanation. It might well bethe case that to explain the token effects under theirmacroscopic description requires equally macroscopic appealto molecular geometry (where a given geometric shape is multiplerealizable by distinct spatial arrays of atoms). But such explanatoryirreducibility is, as we’ve seen, the hallmark of forms of weakemergence.

Jonardon Ganeri (2011) also advances an account with affinities toGillett’s. On this account, in virtue of entering into certainconfigurations, elements are “transformed” in that theyacquire new causal powers. He does not speak of “noncausaldownward determination” from a configurational state to thecomponent powers, though the view seems materially equivalent toGillett’s in this regard. The account differs, however, inmaintaining that in the evolving dynamic that follows the appearanceof such new powers, there come to be emergent macro-states thattogether with the transformed micro-entities determine subsequentmicro- and macro-states. The resulting account is thus a hybrid ofGillett’s view and the more familiar novel causal power of thewhole view we saw in the previous section.

Sydney Shoemaker (2002, 2007), while agreeing with Gillett and Ganerithat emergence occurs within special kinds of organized objects orsystems and features novel component powers, dispenses with anydownward determination relation, causal or otherwise. But these powersare novel in a qualified way:

The component entities have powers that, collectively, determine theinstantiation of the emergent property when they are combined in anemergence-engendering way. But these being cases of emergence, thesecannot all be powers that manifest themselves when the components arenot combined in emergence-engendering ways. Some of them must be“latent” powers. Or, since these powers do not remainlatent when their possessors are combined in emergence-engenderingways, let us speak of them as “micro-latent” powers. Wecan contrast these with the “micro-manifest” powers whichthese same entities manifest when they are not combined with otherentities at all, or are configured in ways that are notemergence-engendering. (Shoemaker 2007: 73)

The thesis that newly manifest powers are “latent” willappeal to those who both take causal power to be a primitive featureof fundamental reality and see it as fundamental to all explanatorylinkages between earlier and later stages in the evolution of things.(We saw in the previous section that O’Connor 1994 andO’Connor & Wong 2005, who share these commitments, postulatethat emergent and fundamental macro-states are “upwardly”determined by micro-structural states manifesting latent dispositionsof fundamental individuals to jointly cause such states when soconfigured.)

4.2.4 Transformed elements independent of configured wholes

Finally, several recent authors have proposed “transformationalemergence” accounts that severs the idea of a privileged classof hierarchically-organized wholes from the concept altogether. Theyadvance an ontological framework in which basic and structuredindividuals undergo fundamental change, acquiring new powers that arenot “latent” (in the sense of there being an antecedent,ontologically-grounded disposition for their subsequent appearance)and perhaps losing others. With the advent of new powers, there arenew laws describing their evolution.

Santos (2015a,b) promotes such a view under the banner of“relational ontology”. The dynamic itself is constantlyevolving, as elements are transformed through interactions with otherelements.

[W]hole is just aword that names the relational totality ofthe individualrelata and their relations, [which are] theonly real causal agents…. (2015a: 28)

Santos suggests that such a view of things is indicated by moderncellular and developmental biology, but the theoretical descriptionshe cites (e.g., 2015a: 30–32; 2015b: 439–440) appear to beconsistent with the other metaphysical frameworks described in theprevious two sections. The issue of whetherenduringstructured entities are essential or inessential to the broadframeworks embraced by the special sciences is a large issue thatcannot be discussed here.

Humphreys (2016), while not committing to the ubiquity of“transformational emergence”, takes the account a stepfurther away from those emphasizing structured wholes by makinginteractions inessential:

Transformational emergence occurs when an individuala that isconsidered to be a fundamental element of a domainD transformsinto a different kind of individuala*, often but not always asa result of interactions with other elements ofD…Theypossess at least one novel property and are subject to differentlaws…. (2016: 60)

As an intuitive (though perhaps not realistic) example, he notes thatpeople undergo significant temporary psychological change when theyare caught up in the interactions constituting a mob. His centralrealistic example comes from the Standard Model of particle physics,which describes partless muons as very quickly “decaying”into electrons, electron neutrinos, and muon neutrinos (2016:66–67). Here there is fundamental change apart from anytriggering interactions, and not merely a change within individualsbut changeof individuals from one kind to others. Humphreysfurther notes that his earlier “fusion” account ofemergence (1997) is a special case of transformation. When fusionoccurs, basal entities or certain of their properties are lost whenthey fuse with others in producing a unified whole (2016: 74–5).(For accounts similar to Humphrey’s, see Guay & Sartenaer2016 and Sartenaer 2018.)

Transformational accounts clearly stretch the classical concept (whichinforms the taxonomy of the present article). The common thread issimply that of fundamental but lawlike change in the observablepatterns in physical reality through time. For purposes of assessinghow competing accounts fare in characterizing the range of empiricalphenomena uncovered in the sciences, it may be necessary for greaterterminological regimentation to emerge.

4.3 Strong emergence: from property to substance dualism?

Emergentists of all varieties standardly are physical substancemonists about the natural world: all worldly (natural or artifactual)entities are composed or otherwise “made of” entities thatwould be described in a completed fundamental physics, whetherphysical particles, fields, strings, or something else. This view iscommon enough among emergentists that some influential theorists tookit to be a defining element of the doctrine. One might maintain,consistent with substance monism, that wholes exhibiting stronglyemergent, efficacious properties are fundamental, albeit compositeobjects or systems, on the grounds that quantification overthem is required for a minimally adequate account of the world’sdynamics. This might also give rise to an objective basis for identitythrough time, even for organisms undergoing constant change of parts(see O’Connor & Jacobs 2003).

But one can argue that strong emergentism, at least with respect tosome or all mental states, in fact requires a form of substancedualism. On a biological view of emergent thinkers, the micro-physicalboundaries of such thinkers may inevitably be vague, for empiricalreasons. But it is perhaps doubtful thatfundamental causallaws associated with strongly emergent properties would referencevague conditions. The sole apparent alternative is that the propertiesare instantiated in a distinct, non-vague object instead, as anon-physical mind would be. (See Zimmerman 2010 and Hasker 2016 and,for a reply, O’Connor 2016.)

One might also argue for an emergentist form of substance dualism asnecessary to account for (what many strong emergentists regardingconsciousness accept) there being unified subjects of consciousexperience (Nida-Rümelin 2007) or it being an objective matterwhether a conscious subject does or does not survive certain kinds ofradical change (Swinburne 2013). Finally, many general accounts of thecategories of substance/object or physical substance/object inparticular entail that if an object has non-physical properties, itwill fail to count as a composed or physical substance (Francescotti2001, Schneider 2012, and O’Connor 2018.)

Supposing that strong emergence does bring a new substance in itswake, the spirit if not the letter of the usual emergentist commitmentto substance monism is maintained by the weaker constraint that no“higher-level” substances or subjects “floatfree”, actually or modally, from their dependence bases. It isfor just this reason that both Lowe (2008) and Nida-Rümelin(2007) characterize their substance dualism as“non-Cartesian”.

5. Weak vs. Strong Emergence: Contested Phenomena

Certain phenomena and theoretical considerations motivating somecontemporary theorists to endorse a strong emergentist construal ofthose phenomena. (It bears emphasis that strong emergentists typicallyalso suppose that some interesting organized behavior is moreplausibly understood either reductively or as weakly emergent: theirstrong emergentism is piecemeal.) This final section summarizes thosephenomena and considers in general terms the different ways strong andweak emergentists might go about treating them.

5.1 The conscious mind

The conscious mind in its different aspects has long seemed to many toresist plausible ontological characterization in any physical terms,whether reductive or non-reductive (i.e., weak emergentist). (For afairly comprehensive overview, see Chalmers 1996.)

5.1.1 Conscious awareness and its unity

Consider, for a start, our apparently directawareness of ourown conscious experiential states. (Conscious cognitive states such asbeliefs and desires have complicating aspects that cannot be exploredhere. But seesection 5.1.2 immediately below for a possible linkage between the two germane tostrong emergentist conjecture.) The natural physicalist strategy forcharacterizing such awareness would be in terms of a distinctive formof informational flow among distinct physical states (the experientialstate itself and a distinct state of awareness of the experience); yetany such account would seem consistent with thepossibilityof a nonstandard causal source of the state of awareness of theconscious state, such that the subject is utterly mistaken about itsobject, the subject’s current conscious state. Such strongfallibilism about the contents of consciousness seems implausible, asis shown by the fact that philosophers such as Descartes who entertainradical skeptical doubts about much of human knowledge do nottypically extend it to our grasp of our current conscious experience.Is it, for example, conceivable that I am onlyseemingly (butmistakenly) aware of undergoing excruciating pain as of being laid outon the torturer’s rack, while in fact I am in a serene consciousstate as of lying on a warm sandy beach? It seems not, but it is hardto account for this datum in physicalist terms.

Also puzzling for physicalism is theunity of consciousexperience: the fact that our experiences engaging distinct sensorymodalities and our conscious thoughts, moods, and feelings cometogether as aspects of one overall conscious state of a singleconscious subject. On a plausible physicalist account, each of theseaspects will be realized in distinct, physically-separated neuralnetworks, but as yet there is no worked-out physicalist strategy forcapturing the unity of these aspects in experience.

In recent decades, highly general functional characterizations ofconscious awareness have been proposed, notably: higher-ordertheories, on which a state is conscious just in case it stands in theright kind of relationship to a higher-order state that represents it(Carruthers 2000; Rosenthal 2005;entry on higher-order theories of consciousness); global workspace theories, on which a state is conscious just in caseits content is globally accessible to multiple cognitive subsystems(Baars 1997, Dehaene et al. 2006); and integrated information theory(Tononi & Koch 2015), on which a system is conscious just in caseit carries more information than the sum of its parts (and is moreconscious the greater the amount of such “integrated”information). Anti-physicalists have argued that the first of thesetheories implicitly denies or fails to adequately capture the twofeatures of direct awareness and unity; the second and third arehighly suggestive frameworks for theorizing about dynamical aspects ofconsciousness, but insofar as they are developed in purely neuralterms, they seem to provide only a weaker surrogate for what weintrospectively grasp concerning the unity of consciousness, anddon’t speak clearly to the direct awareness relation.

That said, it is worth noting that these features, while present instandard conscious experience, are arguably absent or diminished inunusual clinical cases, and in particular in “split-brain”patients. (See Bayne 2008 and Schechter 2018 for philosophicalexploration of clinical accounts regarding the reports of split-brainpatients.) This suggests a physicalist strategy of using these unusualphenomena as a tool for chipping away at the ostensiblephenomenological basis for rejecting the possibility of an eventualphysicalist account of consciousness. (Seeentry on the unity of consciousness, Sect. 4 for a survey of such disorders of unified consciousness anddiscussions of their possible implications.) An alternative, perhapscomplementary physicalist strategy is to argue that certain seemingaspects of consciousness are simplyillusions which can beexplained in terms of inevitable organizational tradeoffs in theconstruction of finite minds (see Pereboom 2011 and Chalmers2018).

5.1.2 The qualitative and intentional character of mental states

Even more than the nature and unity of conscious awareness, certainseeming intrinsic characteristics—so-called“qualia”—of conscious states have seemed to many torequire an anti-physicalist ontological account. The way a patch ofredness appears in one’s visual field, or the sound of a trumpetnote in one’s auditory field, do not seem to admit ofcharacterization in terms of neural processes within the visual andauditory cortex. Rather, such experiential qualities seem to bedistinctive in kind, engendering a mismatch with thephysical-structural or functional kinds posited by physical theories,and to have a degree of simplicity that is incongruent with themassive physical complexity of the associated physical processes. (Fortwo much-discussed arguments from these apparent characteristics ofconscious experience to at least a modest form of physical-mentaldualism, see Jackson 1982 andentry on qualia and the knowledge argument; and Chalmers 1996 andentry on zombies.)

Some accept this claim regarding these “phenomenal”qualities of conscious experience, but think that there is nocorresponding deep challenge to physicalism with regards to theintentional properties of experiences, representing what theworld around us is like (again, Jackson 1982 and Chalmers 1996). (Thislimited anti-physicalism most lends itself to—although it doesnot entail—the epiphenomenalist version of strong emergencediscussed in4.2.1 above, as it is consistent with the claim that information-processingintentional states are the only mental states that causally contributeto the individual’s future mental and physical states.) However,one may question the independence of phenomenal and intentionalproperties (see, e.g., Horgan & Tienson 2002), and some go so faras to conjecture that intentional properties are in some wayconstituted by phenomenal properties (Mendelovici 2018, Woodward2019). If some such connection thesis is correct, then not onlyexperiences, but other cognitive states such as belief and desire mayresist purely physical characterization (seeentry on phenomenal intentionality).

5.1.3 Conscious will & agency

Human and other animals are not only experiencers and knowers but alsodoers. (The nature of non-human animal choice or volition and theirconcomitant experience of action is here set aside, as it is difficultto conjecture concerning it with any confidence on present knowledge.)Many take Aristotle to have expressed a truism when he said that thechoice of how to act is “up to us”, unlike the inevitableunfolding of causal processes lacking in rationality or other sourcesof deliberative choice. It is widely claimed that the human experienceof conscious, deliberate choice (“willing”) is of direct,“top down” control over one’s action by the agent(see, e.g., O’Connor 2000: Ch.1&4; for an overview ofdiscussions regarding agential experience, see Bayne 2017 and Woodwardforthcoming). At least sometimes, we seem to ourselves not to bemerely loci in which a host of psychological and merely physicalinfluences converge and resolve themselves in behavior, but rather toexercise a power to decide which among the options we are consideringwe shall take, thereby determining whether and how these influenceswill be acted upon.

Such agential experience, if veridical, might indicate that theability to freely choose is strongly emergent. On one account, freechoice manifests a distinctive power that, unlike typical powers, isneither causally “triggered” by a stimulus condition noris made more or less probable by antecedent factors (Lowe 2008, 2013).Alternatively, the kind of power agential experience represents isconsistent with non-determining causal influences that collectivelyconfer objective probabilities on possible choices (O’Connor2008). On either understanding, conscious control over one’s ownactions through exercising the power of choice is at odds with aphysicalist conception on which all systemic behavior is fixed by thearrangement and activity of the system’s parts. Coupled with theplausible position that mental states of whatever variety depend onlower-level physical states, such a result might be seen as providingsupport for taking free will to be a strongly emergent power.

Agential experience is defeasible, and cognitive and social psychologyand more recently neuroscience have been probing the determinants ofchoice and its relationship to conscious awareness. One particularform of study has been taken by many to undermine the veridicality ofthe experience of conscious control over arbitrary choice. Libet 1999and more sophisticated successor studies aim to compare theself-reported time of occurrence of conscious choice with the time ofoccurrence of certain brain activity (a steadily increasing“readiness potential”) that Libet took to be associatedwith the production of the subsequent action. He interpreted theresults as indicating that the behavior was unconsciously initiated bythat brain activity justprior to the time of thesubject’s awareness of having so chosen, and hence that theagent’s sense that she consciously initiated the action isillusory.

Philosophers have roundly criticized the “no free will”interpretation of these studies offered by Libet and some otherneuroscientists (see Mele 2009 for a thorough discussion and Caruso 2012: 189ff, for a dissent from theconsensus). Many have emphasized that the “choices”Libet’s subjects are asked to make depart from paradigmaticallyfree cases in important ways. Some note that the ability of subjectsto precisely time their own conscious events of any kind (includingsimple perceptual cues) have been shown to be not fully reliable:their estimates can be manipulated by conscious primes before andafter the target event. (They also come in degrees of intensity, suchthat the onset of an event that might quickly intensify or otherwiseevolve might be more difficult to determine than the event itself.)Others observe that the studies do not even purport to identify adetermining physical antecedent to conscious choice. Finally,other recent studies (beginning with Schurger et al. 2012) have calledinto question just what kind of neural activity Libet-style studiesare tracking, and in particular whether it is even involved in theproduction of choice and subsequent behavior. All this remains asubject of continued scientific study and debate. (A large-scaleproject is currently underway that is developing a wide array ofexperimental probes of conscious will and its physical determinants:< https://neurophil-freewill.org/> For an overview ofphilosophical and empirical issues concerning human free will, seeentry on free will.)

5.2 Physical Science

5.2.1 Quantum Entanglement

A striking feature of quantum mechanics is known as “quantumentanglement”. When two (or more) quantum particles or systemsinteract in certain ways and are then (even space-like) separated,their measurable features (e.g., position and momentum) will correlatein ways that cannot be accounted for in terms of “pure”quantum states of each particle or system separately. In other words,the two need to be thought of as a coupled system, having certainfeatures which are in no sense a compositional or other resultant ofindividual states of the system’s components (see Silberstein& McGeever 1999 and entries onholism and nonseparability in physics andquantum entanglement and information). Humphreys (2016) construes this as an instance of emergent fusion (section 4.2.4). Insofar as these features have physical effects, they indicate anear-ubiquitous failure of whole-part property supervenience at a verysmall scale. However, it should be observed that quantum entanglementdoes not manifest a fundamental novelty in feature or associatedcausal power, as it concerns only the value or magnitude of afeature/associated power had by its components. (Correlated“spin” values, e.g., are permutations on the fundamentalfeature of spin, rather than being akin to mass or charge as whollydistinctive features.) As such, it does not fit the criteria of manyaccounts of strongly emergence. It is, however, relevant to theepistemic status of such accounts: if one thinks that the existence ofstrong emergence is implausible on grounds that a kind of strong localsupervenience isa priori very plausible for composed systemsgenerally, then the surprising phenomenon of quantum entanglementshould lead you to be more circumspect in your assumptions regardinghow complex systems are put together.

5.2.2 Quantum Chemistry

The interface of physics with chemistry has long been thought tosupport ontological reductionism (or physicalism, more generally).This sentiment goes back to an early declaration by Paul Dirac, one ofthe founding figures of quantum mechanics:

The underlying laws necessary for the mathematical theory of a largerpart of physics and the whole of chemistry are thus completely known,and the difficulty is only that exact applications of these laws leadto equations much too complicated to be soluble. (1929: 714)

In recent decades, there has recently been reconsideration of thisperspective. Hendry (1998, 2010, 2017, 2019), following Woolley (1978,1998) and Primas (1981), contends, to the contrary, that molecularstructure is (or may well be) strongly emergent, in failing to bewholly determined by the arrangements of a molecule’s quantumconstituents; isomers, which have distinct molecular structure yet thesame subatomic constituents, are a particular focus in thesediscussions. Scerri (2012) criticizes this line of thought, suggestingthat it overlooks the possibility that a molecule’squantum-mechanical interaction with its environment may contribute tofixing its overall structure. It should be noted that the details ofScerri’s counter-proposal depend on a particular(collapse-based) interpretation of quantum mechanics. There has been asteady uptick of discussion of this issue among philosophers ofchemistry, and it appears to be unresolved at present. This in turnsuggests that, contra Dirac, strong emergence even at the interface ofphysics and chemistry remains a live possibility. (For furtherdiscussion, see Hettema (2015) and entry onphilosophy of chemistry, Section 6.)

5.2.3 Spacetime

Fundamental physics encompasses two successful theories, GeneralRelativity (GR) and Quantum Field Theory (QFT), that are mutuallyinconsistent. In recent years, a number of Quantum Gravity theorieshave been put forward as candidate deeper theories that can accountfor core dynamical principles and successful predictions of boththeories within one consistent framework. Strikingly, in almost allsuch theories, spacetime is not part of the fundamental ontology, itbeing supplanted by a framework that lacks at least some of itsstructure. Many theorists say that the spacetime of GR is therebyrendered emergent. Its dependence is reflected in how the many rolesthat spacetime plays in GR and the spatiotemporally structuredexperimental observations themselves (and indeed, ubiquitousperceptions of everyday life!) are grounded in nonspatiotemporalstructures in the proposed fundamental theories. Its autonomy takesthe form of nonfundamental spacetime’s being qualitativelydifferent from any feature of the fundamental theory.

The issues raised by such radical proposals are complex, and it iseven debated whether they are empirically coherent, given the rolethat space and time play in our understanding of physical causationand empirical observation, and hence of empirical evidence itself. Forpresent purposes, the key point is that such theories proposesystematic theoreticalreductions, akin to the reduction ofthermodynamics to statistical mechanics, with a concommitantontological reduction of the structures of the reduced theories. Inthis, the case appears unlike the relationship between physics andbiology as weak emergentists see it, in which biological entitiesappear to be special kinds of organized physical structures thatexplain equally distinctive phenomena that are unexplained by morefundamental theories. For a non-technical introduction to some of thetheories at issue and the case for describing them as entailingspacetime emergence, see Wüthrich (2019). For more generalphilosophical reflection on our ordinary concepts of space and timeand their compatibility with theories that treat them asnonfundamental, see Chalmers (forthcoming).

5.3 Pluralist interpretations of the general ontology of the sciences

Champions of physicalism see the marked successes of the physical andbiological sciences in the twentieth and twenty-first centuries asestablishing, at a minimum, that the characteristic processes at alllevels of these domains are fixed by fundamental physical states andprocesses. However, some draw a different conclusion from theseadvances, seeing new bases for strongly emergentist perspectives.

Many sciences treating complex systems of one or other variety haveundergone extensive and impressive development in the last fiftyyears. Theorists in these disciplines freely use terms such as“emergence” and “top-down causation” todescribe the phenomena they study. It is often unclear, however,whether they are best interpreted as describing weakly or stronglyemergent states and associated behavior. Among those who havereflected recently on the state of their discipline, solid statephysicist Laughlin (2005), systems biologists Noble (2006) and Boogerd(Boogerd et al. 2005, 2007), and neuroscientist Walter Freeman (1999,2000) and cosmologist Ellis (2016) appear to advocatestrongly emergentist accounts of aspects of solid statephysics, biological life, and the human mind, respectively. They arguethat the pertinent sciences encompassing these domains give equal,complementary places to “bottom up” and “topdown” determinants and principles, and argue that we cannotunderstand the latter as themselves ultimately fixed by instances ofthe former. The general thrust of their detailed arguments is thatthere is no fully “bottom up” way to describe, let aloneexplain, certain organized phenomena. See also Ellis, Noble, andO’Connor (2012) for a collection of state-of-the-art overviewsof the interaction of top-down and bottom-up principles in numerousscientific domains, from fundamental physics to sociology.

Finally, philosopher Nancy Cartwright (1983, 1994, 1999) has defendedover several decades a thorough-going “patchwork” orpluralistic understanding of the relation between the sciences, as analternative to the sort of “physics-at-the-bottom”,asymmetrical hierarchy common to weak emergentists and reductionistsalike. She argues that close consideration of scientific practicesuggests that fundamental physical laws hold only in certainartificial contexts limited to small-scale systems, which systems aremaintained by macroscopic barriers and mechanisms serving to screenoff “downward” causal effects. She suggests that suchcarefully contrived results can give us no reasonable confidence in avision on which all of natural reality is fixed by the unfolding ofbottom-up physical laws alone. While arguing in somewhat differentways, Dupré (1993) is sympathetic to the general pluralist or“anti-fundamentalist” thrust of Cartwright’sperspective. More generally, if anything like Cartwright’s viewis correct, and the right ontological characterization of theunderlying pluralism is one according to which there is fundamentalnovelty across the spectrum of natural reality, one might naturallytake strong emergence to be much more prevalent than even itsproponents have usually assumed. This perspective has affinities tothe “transformational” accounts of emergence noted insection 4.2.4. For discussion of the broad issues raised by Cartwright, see Cat(1998) and entry onthe unity of science, Section 5.

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O’Connor, Timothy and Hong Yu Wong, “Emergent Properties”,Stanford Encyclopedia of Philosophy (Summer 2020 Edition),Edward N. Zalta (ed.), URL = <https://plato.stanford.edu/archives/sum2020/entries/properties-emergent/>. [This was the previous entry on this topic in theStanfordEncyclopedia of Philosophy — see theversion history.]

Acknowledgments

The author acknowledges the substantial contributions to thecomposition of this entry by Jessica Wilson. The entry was originallyplanned and drafted as a fully co-authored work, but Wilson needed tobow out late in the process. After that point, it was heavily revisedand edited, and some new material was added, so she is in no wayresponsible for any flaws in the final product. Even so, the author isindebted to her.

This publication was made possible through the support of a jointgrant from the John Templeton Foundation and the Fetzer Institute. Theopinions expressed in this publication are those of the author(s) anddo not necessarily reflect the views of the John Templeton Foundationor the Fetzer Institute.

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