1. Introduction
The concept of affordance is the central theoretical construct of ecological psychology. It was introduced in its full-fledged form by James J. Gibson (Reference Gibson1979) to account for what human and non-human animals perceive. The notion of affordances has recently been invoked in philosophical theories of mental content (e.g., Rowlands Reference Rowlands1997; Bermúdez Reference Bermúdez1998), but has yet to be submitted to sustained conceptual analysis. Notably, ecological psychologists themselves consider the notion of affordances still poorly defined (e.g., Stoffregen Reference Stoffregen2000). What I propose in this paper is firstly a critical interpretation of Gibson's theory of affordances, and secondly an analysis of the peculiar dispositional nature of affordance properties. My ultimate objective is to provide a general framework for using the concept of affordance in the context of philosophical projects that may not share some of the assumptions and theoretical objectives of ecological psychology.
2. Gibson on Affordances
The concept of affordance is a theoretical concept, introduced by Gibson (Reference Gibson1979) to play a role he believed no existing concept played in psychology:
The affordances of the environment are what it offers the animal, what it provides or furnishes, either for good or ill. The verb to afford is found in the dictionary, but the noun affordance is not. I have made it up. I mean by it something that refers to both the environment and the animal in a way that no existing term does. It implies the complementarity of the animal and the environment. (Gibson Reference Gibson1979, 127; emphasis in original)
The affordances of the environment, Gibson suggested, are the offerings of the environment, roughly the sets of threats (negative affordances) and promises (positive affordances) that characterize items in the environment relative to organisms. Gibson sometimes used a characteristic linguistic construct to refer to affordances, namely, [verb phrase]-able. For example, he described a surface such as the brink of a cliff as fall-off-able, a substance such as an apple as eat-able, an object such as a stone as throw-able, an animal such as a conspecific as copulate-with-able, an event such as a burning fire as cook-with-able. In each case, the affordance property is possessed by a bearer relative to a specific organism or class of organisms.
Gibson introduced affordances to capture what he took to be the essential complementarity between organisms and environment. He believed such complementarity could be lost in two different ways, exemplified respectively by the languages of physics and phenomenology. Physics employs basic explanatory properties such as mass, charge, spin. These are paradigmatic objective properties, namely, properties instantiated independently of any actual or potential response to the property bearer on the part of organisms. Every organism, Gibson thought, is a perceiver and a behaver, but “this is not to say that it perceives the world of physics and behaves in the space and time of physics” (Gibson Reference Gibson1979, 8). What the organism is a perceiver of, and a behaver in, are environments (or niches). Gibson's idea is that the organism and the environment make an inseparable pair, where each term has to be understood relative to the other.
This inseparability brings along two desiderata for theories of perception and action. Firstly, the size-level of the physical world in which environments are carved must be the intermediate one between the cosmic and the atomic. Organisms perceive and behave in a world of objects measured in the scale of their body sizes and engage in activities occurring at the temporal scale of biological processes. Secondly, the explanatory properties devised to understand perception and action must have a relational nature, where the two terms of the relation grounding the property refer respectively to the organism and to the environment.
One way to discharge the two desiderata is the one favored by phenomenologists. Gibson did not define phenomenology, but indicated the Gestalt psychologists Kurt Lewin and Kurt Koffka as representatives of a relational approach he wanted to improve upon. Lewin (Reference Lewin1935) and Koffka (Reference Koffka1935) described objects in the environment in terms of what organisms can do with them. For instance, Koffka ascribed to a postbox the relational property of having a “letter-mailing” demand character. However, Gibson took phenomenologists to assume that the demand character is “bestowed upon [the postbox] in experience, and bestowed upon it by a need of the observer” (Gibson Reference Gibson1979, 138).
Instead, he wanted to define affordances so as to be able to say that the postbox is letter-mailing-with-able relative to an organism O independently of whether or not it is perceptually experienced as such, and independently of whether or not, once perceived, it is attended to by O (this will be contingent upon O's needs). As he put it, “an affordance is not bestowed upon an object by a need of an observer and his act of perceiving it” (Gibson Reference Gibson1979, 139). What bestows affordances upon items in the environment, Gibson suggested, are a set of real, or objective, or physical properties of the affordance bearer and the organism that are relevant to make a specific behavioral activity possible (e.g., grasping, catching, being eaten by). For example, Gibson (Reference Gibson1979, 133) indicated that “to be graspable, an object must have opposite surfaces separated by a distance less than the span of the hand.”
The properties that ground the threats and promises associated with affordance bearers will have to be studied by a new ecological physics, the explananda of which are perception-action couplings performed by organisms in environments. Some Gibsonians take the relevant physical properties to be relations between sizes of the body and sizes of the affordance bearer (e.g., Warren Reference Warren1984). But it is clear that this is only a preliminary step in the attempt to ground affordances into the physical world.Footnote 1 As I will argue in the next section, an item X is graspable relative to O in virtue of O's ability to grasp X, and such ability does not consist merely of the presence of a physical relation between the distance of X's “opposite surfaces” and O's “span of the hand” (e.g., the span of the hand of a person with a paralyzed open hand is unchanged, but the ability to grasp is lost).Footnote 2 The inadequacy of mere body-scales to capture abilities becomes even more evident when the ability is, say, that of mailing a letter, or that of scoring a point.
We can now turn to some puzzling remarks Gibson made about the ontological status of affordances:
An important fact about the affordances of the environment is that they are in a sense objective, real, and physical, unlike values and meanings, which are often supposed to be subjective, phenomenal and mental. But, actually, an affordance is neither an objective property nor a subjective property; or it is both if you like. An affordance cuts across the dichotomy of subjective-objective and helps us to understand its inadequacy. (Gibson Reference Gibson1979, 129)
What Gibson wanted to say here, I take it, is that affordances are not subjective in the sense that they are bestowed upon the world in virtue of an actual or potential perceptual experience of a subject. In this respect, they are different from paradigmatic secondary properties such as colors, namely, properties that are instantiated by virtue of a sensory response (a color experience) a subject would have relative to the property bearer, in suitable circumstances. However, I would argue that affordances are similar to secondary properties with respect to their being response-dependent, namely, instantiated by virtue of a behavioral response (e.g., a catching, a throwing) a subject would have relative to the property bearer, in suitable circumstances. In the next section, I try to capture the nature of this response dependency by unveiling the implicit dispositional nature of affordances. According to the view I propose, the sense in which affordances are also objective is the sense in which a disposition in good standing has a basis constituted by objective properties.
In this paper, I remain neutral on the issue of whether affordances really are the dispositional properties I describe in the next section (subjective in the sense of response dependent) or, rather, the bases of such properties (objective in the sense of response independent). I just remark that, however we settle the ontological dispute, affordances represent a significant novelty with respect to the standard Lockean tri-partition between primary, secondary, and tertiary qualities.Footnote 3
Before turning to the clarification of the dispositional nature of affordances, we need to focus on their relation to perception. Here are two relevant quotes on the matter:
[P]laces, attached objects, objects, and substances are what are mainly perceived, together with events, which are changes of these things. To [perceive] these things is to perceive what they afford. (Gibson Reference Gibson1979, 240)
The meaning or value of a thing consists of what it affords. (Gibson et al. Reference Gibson, Reed and Jones1982, 407)
The crucial empirical hypothesis of ecological psychology is that affordances are perceivable. Since affordances constitute, as claimed in the second quote, ecological meanings or values, the crucial hypothesis can be reformulated by saying that ecological meanings/values are perceivable. For example, to perceive the ecological meaning of a fire would mean to perceive the fire as cook-with-able, or burning-by-able, etc. For Gibson, to perceive is to pick up information for purposes of behavioral discrimination (rather than for purposes of belief fixation). Gibson emphasized that this pick-up results from active exploration of the environment, that it “can sometimes occur without the accompaniment of sense impressions” (Gibson Reference Gibson1966, 2), and that is always accompanied by perception of one's own body (proprioception).
But how do organisms pick up information? To pick up information, argued Gibson, is to become attuned to invariants and disturbances that specify to-be-perceived properties. An intuitive understanding of these technical notions is the following. An invariant is a property of the structure of ambient energy arraysFootnote 4 (e.g., the optic array, the acoustic array, etc.) instantiated when, relative to some source of change such as a moving point of observation or a moving source of illumination, the structure is left unchanged in a way that is typical of the item specified (e.g., a reflectance can specify the substance “coal” by being unchanging in the way characteristic of coal substances). A disturbance is a property of the structure of ambient energy arrays instantiated when, relative to some source of change (e.g., the change constituted by an approaching predator), the structure presents a pattern of change that is typical of the item specified (e.g., the contour of an animal can specify the event “approaching predator” by changing in the way typical of approaching predators).
In general terms, to say that affordances are perceivable is to say that there are invariants and disturbances in ambient-energy arrays that specify the threats and promises of items in the environment. For example, to say that the eat-ability of a given apple is perceivable, or that the being-hit-by-ability of a flying ball is perceivable is to say that there is a sensory appearance—a way to be visible/audible/tangible/odorous/tastable—typical respectively, of apples affording eating, and of flying balls affording being hit by. Gibson was very clear that we cannot establish “a priori” what affordances are specified in ambient energy.Footnote 5 As he put it, “[t]he central question for the theory of affordances is not whether they exist and are real but whether information is available in ambient light for perceiving them” (Gibson Reference Gibson1979, 140). Information is available for perceiving all and only those offerings of the environment that are associated with typical sensory appearances. In some cases, the organism will have to learn to perceive a perceivable affordance, that is, learn to become attuned to the invariant or disturbance specifying it. What invariants and disturbances are, in fact, available for the specification of affordances will have to be established by empirical investigation.
The most controversial part of ecological psychology has to do with the way in which Gibsonians cash out the notion of becoming attuned to invariants and disturbances. On the one hand, they assume that invariants and disturbances carry information about affordances by lawfully specifying them (see, for example, Turvey et al. Reference Turvey, Shaw, Reed and Mace1981, 267–275). The idea is that a law of nature links properties of the structure of ambient-energy arrays and affordances. On the other hand, Gibsonians think that the presence of a lawful specification is such that the assimilation of information for affordances can occur without the involvement of mental representations and mental processes involving representations (e.g., inferences, computations, retrievals of memories, etc.). (See, for example, Gibson Reference Gibson1979, 238–263.) This is what lies behind the trademark claim of ecological psychologists that perception is direct.
I think these two related moves are highly questionable.Footnote 6 However, I won't argue for this point here. As I pointed out, affordances are what they are independently of whether or not they are perceivable (some may not be), and independently of how they are eventually perceived (directly or indirectly). At the same time, the relevance of affordances for the explanation of behavior crucially hinges upon their perceivability. Before reflecting on the relation between affordances and perception, we need a more thorough understanding of what it means to ascribe an affordance to a bearer. Once we have decided what kinds of properties qualify as affordances, we can then discuss whether, relative to some theory of perception T, such properties are perceivable. One of the objectives of my paper is to make affordances available as potential objects of perception for theories of perception that may not share Gibson's radical rejection of mental representations and mental processes as useful explanatory constructs.
3. Affordances as Dispositions
3.1. The Semantics of Dispositions
Since I want to argue that affordances are a species of dispositional properties, I need an account of the genus. However, philosophical controversies flourish concerning what properties are, and what makes them dispositional. Given the space constraints on this paper, I can only proceed stipulatively, and refer the reader to some relevant authors who clarify and defend the working assumptions I make.
Firstly, I take a property to be “the set of all its instances—all of them, this- and other-worldly alike” (Lewis Reference Lewis1986, 50).
Secondly, I assume that properties are abundant in David Lewis's sense, namely as “extrinsic, as gruesomely gerrymandered, as miscellaneously disjunctive as you please” (Lewis Reference Lewis1986, 59).
Thirdly, I assume that to clarify the meaning of properties is to clarify the semantics of the predicates (if any) expressing them.
Fourthly, I follow Stephen Mumford (Reference Mumford1998) in thinking that a predicate is dispositional or categorical depending on the way in which its ascription entails subjunctive conditionals.Footnote 7 Let us take two paradigmatic examples from the classes we want to distinguish, namely, fragility and triangularity. The ascription “X is fragile” entails the subjunctive conditional “if X were (suitably) hit, then X would break,” and the ascription “X is triangular” entails the subjunctive conditional “if X's sides were (suitably) counted, the result would be three.”Footnote 8 This shows that we cannot simply draw the distinction by saying that dispositional ascriptions, differently from categorical ones, entail subjunctive conditionals. What Mumford (Reference Mumford1998, 77) indicates as the fundamental ground of difference is that the entailment is by conceptual necessity in the case of fragility, but not in the case of triangularity. Roughly speaking, the idea is that the subjunctive conditional captures what being fragile means, whereas it does not capture what being triangular means (although is captures something that necessarily follows from what being triangular means). To defend Mumford's (Reference Mumford1998) account would of course require a convincing defense of the notion of conceptual necessity. Here, I simply assume that Mumford's way of drawing the distinction can successfully be defended.
Fifthly, I adopt from Elizabeth Prior (Reference Prior1985) the idea that dispositional predicates are best thought of as incomplete predicates. In the case of classic dispositions such as fragility, solubility, flammability, and so on, she suggests that the completer is a set of background circumstances C. In other words, a bearer X is or is not fragile, or soluble, or inflammable not simpliciter, but given C. For example, the very same object X may be, say, fragile given a background temperature of −170°C, and not fragile given a background temperature of 30°C.
3.2. What Kinds of Dispositions are Affordances?
I take predicates such as climbable, reachable, letter-mailing-with-able, bump-into-able, and so on, to be dispositional predicates.Footnote 9 To clarify the semantics of an affordance ascription of the general form “X has affordance property A (at time t relative to an organism O in circumstances C)” is to fill in placeholders in a subjunctive conditional conditional that is entailed by the affordance ascription by conceptual necessity. Such a conditional conditional has the following general form:Footnote 10
At time t, if background circumstances C were the case, then (if a set of triggering circumstances T were the case, then a manifestation M involving X and O would be the case with probability p).Footnote 11
Under this view, affordance predicates are time-indexed incomplete predicates, whose completer is a set of background circumstances referring to an organism at a time in a set of environmental circumstances. For example, a tree X is climbable/not-climbable not simpliciter, but at time t relative to squirrel O in background circumstances C. Such circumstances would include, among other things, that the squirrel O is not surrounded by a structure that prevents his movements, and that the tree X is not on fire.
Two insights concerning C have to be kept in mind from the literature on dispositions. The first is that the background circumstances C under which a disposition is possessed cannot be listed exhaustively, because they consist of an indefinitely large set. In fact, there is always some condition c that could be added to a specified set C such that, given C and c, any object X would no longer possess any given disposition (e.g., there are conditions under which sugar is not water soluble, Ming vases are not fragile, etc.). The second is that the list of background circumstances C cannot be left entirely open ended, because there is presumably always some set C such that, given C, almost any object X would possess almost any disposition (e.g., there are conditions under which bars of steel are soluble, pieces of diamond fragile, etc.).
How should set C be accounted for then? A full discussion would take us too far, but the general idea I endorse is that, for what concerns affordance predicates, we should rely on a tacit understanding of C as the set of normal ecological circumstances. Footnote 12 Let me just mention that a basic desideratum for any theory of normal ecological circumstances is the avoidance of vacuity, which would result, for example, from defining conditions C as ecologically normal just in case nothing interfered with the affordance being possessed. What a theory of normal circumstances has to account for is how to sort, within the class of potential defeaters of affordance manifestations, those that are, and those that are not compatible with the affordance being possessed by X relative to O. In other words, the avoidance of vacuity demands a tacit understanding of the difference between failures of manifestation of a truly ascribed affordance, and evidence that the affordance is falsely ascribed.
The trademark feature of affordances is that their manifestation is always constituted by an event in which the affordance-bearer X and the organism O are both involved.Footnote 13 If we look at Gibson's examples of affordances, we notice manifestations such as climbing, catching, getting under, eating, mailing a letter, but also such as bumping into, getting burned by, falling off, being eaten by. Whereas events in the first list constitute things organisms do, events in the second list constitute things that happen to them.
The distinction is in my view important enough to distinguish between two classes of affordances, namely, goal affordances (their manifestation is a doing) and happening affordances (their manifestation is a happening). What makes an organism-involving event a doing rather than a happening is how the manifestation is related to the triggering circumstances. In the philosophy of action, the doing/happening distinction is commonly cashed out in terms of the distinction between events that respectively are and are not intentional under some description (Davidson Reference Davidson1980, 61). In other words, actions are typically understood as events triggered by an intention, and such that there is a description of the event under which the intention is fulfilled (in the right way). But this distinction can at best help us sort goal affordances from happening affordances relative to human organisms, namely, organisms capable of the kind of conceptual achievement demanded by formulating an intention, for I take intentions to be propositional attitudes beyond the reach of, say, bacteria.
What we need are distinctions that apply at lower levels, namely, at all levels of organismic life relative to which we want our affordance concept to apply. I provisionally distinguish between doings and happenings as events that respectively are or are not goal-achieving under some description. Roughly speaking, doings are events triggered by the selection of a goal, and such that there is a description of the event under which the goal is achieved (in the right way). I leave it an open question for now how to characterize precisely the notions of selecting and achieving a goal in the right way for various classes of organisms, a topic that would demand more attention than I can devote to it here.Footnote 14
Under the view I am proposing, if it is true that, given background circumstances C, an organism O can at t engage in an event that qualifies as a doing or a happening M and involves X, then X is at t an affordance-bearer with manifestation M relative to O in circumstances C.Footnote 15 This being the case, an important determinant of what counts as an affordance property is how we interpret the modal force of “O can at t.” To simplify, I will elaborate the point only with respect to goal affordances (but a similar story can be told about happening affordances).
To establish what an organism O can do at t is tantamount to establishing what O's abilities are at t.Footnote 16 A distinction is for example the one between a current ability—e.g., I am now able to sit—and a latent ability—e.g., I am now not able to juggle, but I could learn. Another distinction is the one between degrees of reliability of current abilities. For example, whereas I am currently highly reliable at sitting when I select it as a goal, I am much less reliable at catching, and I manage to ride a unicycle about once every 10 times I try. The decisions we make with respect to how to count O's abilities at t—what O can do at t—will determine the extension of the class of goal affordances relative to O.
I propose to restrict affordance-bestowing abilities to current abilities of individual organisms. The subjunctive conditional entailed by conceptual necessity by the ascription of goal affordances will then have the form “at t, if background circumstances C were the case, then if organism O were to select the goal of [climbing, grasping, letter mailing, etc.], then the manifestation of [climbing, grasping, letter mailing, etc.] would occur with positive probability p.” According to this view, when we describe a tree X as climb-able for an organism O at t, what we are saying is that, given (tacitly understood) normal ecological conditions, O would succeed at climbing X with some positive probability p were O to try climbing at t. O's latent ability to climb, I suggest, should not be considered sufficient to ascribe climb-ability at t relative to O (but the tree X may become climb-able relative to O at some later time t′, if O's latent ability becomes current at t′).
But is really any positive probability p enough for ascribing affordances? The answer here is open, it all depends on the theoretical uses we want to make of the affordance concept. My inclination is to restrict affordance-bestowing current abilities to reliable current abilities, namely, abilities such that the probability of success given a try is not only positive but also significant (the reliable/unreliable boundary will of course be blurred). But I will not discuss this preference here, and conclude by introducing a terminological distinction between affordances with respect to the degree of reliability of their underlying abilities.
I distinguish between surefire affordances (i.e., affordances such that the manifestation follows the triggering circumstances with certainty), and probabilistic affordances (i.e., affordances such that the manifestation follows the triggering circumstances with some positive probability p less than 1). Sit-ability is now for me a candidate surefire goal affordance, and catch-ability and ride-with-ability are now for me probabilistic goal affordances of different degrees of reliability.Footnote 17 Notice that the surefire/probabilistic distinction applies to both goal affordances and happening affordances. Many of the latter seem in fact to be of a probabilistic variety (e.g., the brink of a cliff possesses the probabilistic affordance of fall-off-ability).
4. Conclusion
In the first part of this paper, I have tried to clarify Gibson's theory of affordances, with special focus on their relational nature, and their status as potential objects of perception. In the second part, I have analyzed the dispositional character of affordances, introducing a few hopefully useful distinctions between kinds of affordances. I now want to conclude by pointing the reader to an opportunity and a risk we face by allowing affordances in our explanatory toolbox.
The opportunity, a tantalizing one in my opinion, is that of acquiring an instrument to account for objects of perception imbued with a kind of meaning—ecological meaning—that we can legitimately suppose to be grasped in a similar way by animals, infants, and adult human beings, at least with respect to some affordances. For example, it seems quite possible that, despite their cognitive differences, an animal, a child, and an adult person may perceive the brink of a cliff as fall-off-able according to a common perceptual process. What I am suggesting is not that they may all similarly perceive that the brink of cliff is fall-off-able, but rather that they may all similarly engage in a perceptually guided behavioral discrimination of the brink of the cliff as fall-off-able.
The risk to be avoided is the assumption that this commonality of perceptual processing extends uniformly across the board. Let us focus on human goal affordances. As I pointed out before, the domain of goal affordances is complementary to the domain of doings or actions individuals can perform. The category of human actions includes mental actions (e.g., dividing a number by two) and physical actions. The latter category is commonly divided between basic physical actions (bodily movements, e.g., catching a ball) and nonbasic physical actions (consequences of bodily movements, e.g., scoring a point by catching a ball).
There is no reason to assume that a theory of perception can explain in the same way how humans perceive, if they do, a number being divide-by-two-able (a mental affordance), a flying ball being catch-able (a basic physical affordance), and a flying ball being score-with-able (a nonbasic physical affordance). Whereas catch-ability seems suitable for being grasped in a similar way by language-using and non-language-using creatures, divide-by-two-ability and score-with-ability demand a story about perception in which the language-mediated ability to master concepts such as divisible number or point in a game will play a prominent role.
This is just to say that affordance properties ought not be treated by default as a homogenous block by theories of perception. They inherit from their constitutive relations with kinds of doings and kinds of happenings a number of distinguishing properties that are potentially relevant to establish whether they are perceivable, and, if they are, how they are perceived.
