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EIGHT

THE THEORY OF

AFFORDANCES

I have described the environment as the surfaces that separate substances fronmedium in which the animals live. But I have also described what the enviromaffords animals, mentioning the terrain, shelters, water, fire, objects, tools, canimals, and human displays. How do we go from surfaces to affordances? And if tis information in light for the perception of surfaces, is there information forperception of what they afford? Perhaps the composition and layout of surfaces co1tute what they afford. If so, to perceive them is to perceive what they afford. Tha radical hypothesis, for it implies that the "values" and "meanings" of things inenvironment can be directly perceived. Moreover, it would explain the sense in w:values and meanings are external to the perceiver.

The affordances of the environment are what it offers the animal, what it pro01or furnishes, either for good or ill. The verb to afford is found in the dictionary,the noun affordance is not. I have made it up. I mean by It something that referboth the environment and the animal in a way that no existing term does. It impthe complementarity of the animal and the environment.' The antecedents of the band the history of the concept will be treated later; for the present, let us consiexamples of an affordance.

If a terrestrial surface is nearly horizontal (instead of slanted), nearly flat (inst,of convex or concave), and sufficiently extended (relative to the size of the animal) ,if its substance is rigid (relative to the weight of the animal), then the surface affo1support. It is a surface of support, and we call it a substratum, ground, or floor. Iistand-on-able, permitting an upright posture for quadrupeds and bipeds. It is therefiwalk-on-able and run-over-able. It is not sink-into-able like a surface of water oswamp, that is, not for heavy terrestrial animals. Support for water bugs is differe:

Note that the four properties listed-horizontal, flat, extended, and rigid-wotbe physical properties of a surface if they were measured with the scales and standsunits used in physics. As an affordance of support for a species of animal, howev1they have to be measured relative to the animal. They are unique fur that animal Thare not just abstract physical properties. They have unity relative to the posture w

127

James J Gibson

The Ecological Approach to Visual Perception

behavior of the animal being considered. So an affurdance cannot be measured as we measure in physics.

Terrestrial surfaces, of course, are also climb-on-able or fall-off-able or get-under­neath-able or bump-into-able relative to the animal. Different layouts afford different behaviors for different animals, and different mechanical encounters. The human spe­cies in some cultures has the habit of sitting as distinguished from kneeling or squatting. If a surface of support with the four properties is also knee-high above the ground, it affords sitting on. We call it a seat in general, or a stool, bench, chair, and so on, in particular. It may be natural like a ledge or artificial like a couch. It may have various shapes, as long as its functional layout is that of a seat. The color and texture of the surface are irrelevant. Knee-high for a child is not the same as knee-high for an adult, so the affordance is relative to the size of the individual. But if a surface is horizontal, flat, extended, rigid, and knee-high relative to a perceiver, it can in fact be sat upon. If it can be discriminated as having just these properties, it should look sit-on-able. If it does, the affurdance is perceived visually. If the surface properties are seen relative to the body surfaces, the self, they constitute a seat and have meaning.

There could be other examples. The different substances of the environment have different affordances for nutrition and for manufucture. The different objects of the environment have different affordances for manipulation. The other animals affurd, above all, a rich and complex set of interactions, sexual, predatory, nurturing, fighting, playing, cooperating, and communicating. What other persons affurd, comprises the whole realm of social significance for human beings. We pay the closest attention to

the optical and acoustic information that specifies what the other person is, invites, threatens, and does.

THE NICHES OF THE ENVIRONMENT

Ecologists have the concept of a niche. A species of animal is said to utilize or occupy a certain niche in the environment. This is not quite the same as the habitat of the species; a niche refers more to how an animal lives than to where it lives. I suggest that a niche is a set of affordances.

The natural environment offers many ways of life, and different animals have

different ways of life. The niche implies a kind of animal, and the animal implies a kind of niche. Note the complementarity of the two. But note also that the environment as

a whole with its unlimited possibilities existed prior to animals. The physical, chemical, meteorological, and geological conditions of the surface of the earth and the pre-existence

of plant life are what make animal life possible. They had to be invariant for animals to evolve.

THE INFORMATION FOR VISUAL PERCEPTION

128

There are all kinds of nutrients in the world and all sorts of ways of getting food; all sorts of shelters or hiding places, such as holes, crevices, and caves; all sorts of materials for making shelters, nests, mounds, huts; all kinds of locomotion that the environment makes possible, such as swimming, crawling, walking, climbing, Rying. These offerings have been taken advantage of; the niches have been occupied. But, for all we know, there may be many offerings of the environment that have not been taken

advantage of, that is, niches not yet occupied. In architecture a niche is a place that is suitable for a piece of statuary, a place into

which the object fits. In ecology a niche is a setting of environmental features that are suitable for an animal, into which it fits metaphorically.

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 alfordance cuts across the dichotomy of subjective-objective and helps us to understand its inad· equacy. It is equally a fact of the environment and a fact of behavior. It is both physical and psychical, yet neither. An alfordance points both ways, to the environment and to the observer.

The niche for a certain species should not be confused with what some animal psychologists have called the phenomenal environment of the species. This can be taken erroneously to be the "private world" in which the species is supposed to live, the "subjective world," or the world of"consciousness." The behavior of observers depends on their perception of the environment, surely enough, but this does not mean that their behavior depends on a so-called private or subjective or conscious environment. The organism depends on its environ'!lent for its life, but the environment does not depend on the organism for its existence.

MAN'S ALTERATION OF THE NATURAL ENVIRONMENT

In the last few thousand years, as everybody now realizes, the very face of the earth has been modified by man. The layout of surfaces has been changed, by cutting, clearing, leveling, paving, and building. Natural deserts and mountains, swamps and rivers, forests and plains still exist, but they are being encroached upon and reshaped

by man-made layouts. Moreover, the substances of the environment have been partly converted from the natural materials of the earth into various kinds of artificial materials

such as bronre, iron, concrete, and bread. Even the medium of the environment-the

129

air for us and the water for fish-is becoming slowly altered despite the restorative cycles that yielded a steady state for millions of years prior to man.

Why has man changed the shapes and substances of his environment? To change what it affords him. He has made more available what benefits him and less pressing what injures him. In making life easier for himself, of course, he has made life harder for most of the other animals. Over the millennia, he has made it easier for himself t o get food, easier t o keep warm, easier t o see at night, easier to get about, and easier t o

train his offspring. This is not a new environment-an artificial environment distinct from the natural

environment-but the same old environment modified by man. It is a mistake to

separate the natural from the artificial as if there were two environments; artifacts have to be manufactured from natural substances. It is also a mistake to separate the cultural environment from the natural environment, as if there were a wodd of mental products

distinct from the world of material products. There is only one world, however diverse, and all animals live in it, although we human animals have altered it to suit ourselves. We have done so wastefully, thoughtlessly, and, if we do not mend our ways, fatally.

The fundamentals of the environment-the substances, the medium, and the surfaces-are the same for all animals. No matter how powerful men become they are not going to alter the fact of earth, air, and water-the lithosphere, the atmosphere, and the hydrosphere, together with the interfaces that separate them. For terrestrial animals like us, the earth and the sky are a basic structure on which all lesser structures depend. We cannot change it. We all fit into the substructures of the environment i n our various ways, for w e were all, in fact, formed b y them. W e were created by the world we live in.

SOME AFFORDANCES OF THE TERRESTRIAL ENVIRONMENT

Let us consider the affordances of the medium, of substances, of surfaces and their layout, of objects, of animals and persons, and finally a case of special interest for ecological optics, the affording of concealmeant by the occluding edges of the environ­ment (Chapter 5).

THE MEDIUM

Air affurds breathing, more exactly, respiration. It also affords unimpeded locomotion relative to the ground, which affords support. When illuminated and fog-free, it affords

THE INFORMATION FOR VISUAL PERCEPTION

visual perception. It also affords the perception of vibratory events by means of sound fields and the perception of volatile sources by means of odor fields. The airspaces between obstacles and objects are the paths and the places where behavior occurs.

The optical information to specify air when it is clear and transparent is not obvious. The problem came up in Chapter 4, and the experimental evidence about the seeing of"nothing" will be described in the next chapter.

THE SUBS TANCES

Water is more substantial than air and always has a surface with air. It does not afford respiration for us. It affords drinking. Being fluid, it affords pouring from a container. Being a solvent, it affords washing and bathing. Its surface does not afford support for

large animals with dense tissues. The optical information for water is well specified by the characteristics of its surface, especially the unique fluctuations caused by rippling (Chapter 5).

Solid substances, more substantial than water, have characteristic surfaces (Chapter 2). Depending on the animal species, some afford nutrition and some do not. A few are toxic. Fruits and berries, for example, have more food value when they are ripe, and this is specified by the color of the surface. But the food values of substances are often misperceived.

Solids also afford various kinds of manufucture, depending on the kind of solid

state. Some, such as Rint, can be chipped; others, such as clay, can be molded; still others recover their original shape after deformation; and some resist deformation strongly. Note that manufacture, as the term implies, was originally a form of manual behavior like manipulation. Things were fabricated by hand. To identify the substance in such cases is to perceive what can be done with it, what it is good for, its utility; and the hands are involved.

THE SURFACES AND THEIR LAYOUTS

I have already said that a horizontal, flat, extended, rigid surface affords support. It permits equilibrium and the maintaining of a posture with respect to gravity, this being

a force perpendicular to the surface. The animal does not fall or slide as it would on a steep hillside. Equilibrium and posture are prerequisite to other behaviors, such as

locomotion and manipulation. There will be more about this in Chapter 12, and more evidence about the perception of the ground in Chapter 9. The ground is quite literally the basis of the behavior of land animals. And it is also the basis of their visual

THE THEORY OF AFFOROANf'J.�

perception, their so-called space perception. Geometry began with the study of the

earth as abstracted by Euclid, not with the study of the axes of empty space as

abstracted by Descartes. The affording of support and the geometry of a horizontal plane are therefore not in different realms of discourse; they are not as separate as we

have supposed. The flat earth, of course, lies beneath the attached and detached objects on it. The

earth has "furniture," or as I have said, it is cluttered. The solid, level, flat surfac_-e

extends behind the clutter and, in fact, extends all the way out to the horizon. This is not, of course, the earth of Copernicus; it is the earth at the scale of the human animal, and on that scale it is flat, not round. Wherever one goes, the earth is separated &om

the sky by a horizon that, although it may be hidden by the clutter, is always there.

There will be evidence to show that the horizon can always be seen, in the sense that

it can be visualized, and that it can always be felt, in the sense that any surface one

touches is experienced in relation to the horizontal plane. Of course, a horizontal, flat, extended surface that is nonrigid, a stream or lake,

does not afford support for standing, or for walking and running. There is no footing,

as we say. It may afford floating or swimming, but you have to he equipped for that,

by nature or by learning.

A vertical, flat, extended, and rigid surface such as a wall or a cliff face is a barrier to pedestrian locomotion. Slopes between vertical and horizontal afford walking, if

easy, but only climbing, if steep, and in the latter case the surface cannot be flat; there must be "holds" fur the hands and feet. Similarly, a slope downward affords falling if steep; the brink of a cliff is a falling-off place. It is dangerous and looks dangerous. The

affordance of a certain layout is perceived if the layout is perceived.

Civilized people have altered the steep slopes of their habitat by building stairways so as to afford ase,-ent and descent. What we call the steps afford stepping, up or down,

relative to the size of the person's legs. We are still capable of getting around in an

arboreal layout of surfaces, tree branches, and we have ladders that afford this kind of locomotion, but most of us leave that to our children.

A cliff face, a wall, a chasm, and a stream are barriers; they do not afford pedestrian

locomotion unless there is a door, a gate, or a bridge. A tree or a rock is an obstacle.

Ordinarily, there are paths between obstacles, and these openings are visible. The progress of locomotion is guided by the perception of barriers and obstacles, that is, by

the act of steering into the openings and away from the surfaces that afford injury. I have tried to describe the optical inlormation for the control of locomotion (Gibson,

1958), and it will be further elaborated in Chapter 13. The imminence of collision with

a surface during locomotion is specified in a particularly simple way, by an explosive rate of magnification of the optical texture. This has been called looming (e.g., Schiff, 1965). It should not be confused, however, with the magnification of an opening

THE INFORMATION FOR VISUAL PERCEPTION

between obstacles, the opening up of a vista such as occurs in the approach to a

doorway.

THE OBJECTS

The affordances of what we loosely call objects are extremely various. It will be recalled

that my use of the terms is restricted and that I distinguish between atta ched objects

and deta ched objects. We are not dealing with Newtonian objects in space, all of which

are detached, but with the furniture of the earth, some items of which are attached to

it and cannot be moved without breakage. Detached objects must be comparable in size to the animal under consideration

if they are to afford behavior. But those that are comparable afford an astonishing

variety of behaviors, especially to animals with hands. Objects can be manufactured

and manipulated. Some are portable in that they afford lifting and carrying, while

others are not. Some are graspable and other not. To be graspable, an object must

have opposite surfaces separated by a distance less than the span of the hand. A five­

inch cube can be grasped, but a ten-inch cube cannot (Gibson, 1966b, p. 119). A large

object needs a "handle" to afford grasping. Note that the size of an object that constitutes a graspable size is specified in the optic array. If this is true, it is not true that a tactual sensation of size has to become associated with the visual sensation of size in order for the affordance to be perceived.

Sheets, sticks, fibers, containers, clothing, and tools are detached objects that afford manipulation (Chapter 3). Additional examples are given below.

1. An elongated object of moderate size and weight affords wielding. If used to

hit or strike, it is a club or hammer. If used by a chimpanzee behind bars to pull in a banana beyond its reach, it is a sort of rake. In either case, it is an extension of the arm. A rigid staff also affords leverage and in that use is a lever. A pointed elongated object affords piercing-if large it is is a spear, if small a needle or awl.

2. A rigid object with a sharp dihedral angle, an edge, affords cutting and

scraping; it is a knife. It may be designed for both striking and cutting, and then it is an axe.

3. A graspable rigid object of moderate size and weight affords throwing. It may

be a missile or only an object for play, a ball. The launching of missiles by supplementary

tools other than the hands alone-the sling, the bow, the catapult, the gun, and so

on-is one of the behaviors that makes the human animal a nasty, dangerous species.

4. An elongated elastic object, such as a.fiber, thread, thong, or rope, affordsknotting, binding, lashing, knitting, and weaving. These are kinds of behavior where

manipulation leads to manufacture.

THE THEORY OF AFFOROANC:FS

5. A h and-held tool of enormous importance is one that, when applied to a surface, leaves traces and thus affords trace-making. The tool may be a stylus, brush,crayon, pen, or pencil, but if it marks the surface it can be used to depict and to write,to represent scenes and to specify words.

We have thousands of names for such objects, and we classify them in many ways:pliers and wrenches are tools; pots and pans are utensi ls; swords and pistols areweapons. They can all be said to have properties or qualities: color, texture, compo­sition, size, shape and features of shape, mass, elasticity, rigidity, and mobility. Or­thodox psychology asserts that we perceive these objects insofar as we discriminatetheir properties or qualities. Psychologists carry out elegant experiments in the labo­ratory to find out how and how well these qualities are discriminated. The psychologistsassume that objects are composed of their qualities. But I now suggest that what weperceive when we look at objects are their affordances, not their qualities. We candiscriminate the dimensions of difference if required to do so in an experiment, butwhat the object affords us is what we normally pay attention to. The special combinationof qualities into which an object can be analyzed is ordinarily not noticed.

If this is true for the adult, what about the young child? There is much evidenceto show that the infant does not begin by first discriminating the qualities of objectsand then learning the combinations of qualities that specify them. Phenomenal objectsare not built up of qualities; it is the other way around. The affordance of an object iswhat the inf.mt begins by noticing. The meaning is observed befure the substance andsurface, the color and form, are seen as such. An affordance is an invariant combinationof variables, and one might guess that it is easier to perceive such an invariant unit

T o PERCEIVE AN AFFORDANCE Is NOT TO CLASSIFY AN OBJECT

The fact that a stone is a missile does not imply that it cannot be other things as well. It can bea paperweight, a bookend, a hammer, or a pendulum bob. It can be piled on another rock tomake a cairn or a stone wall. These affordances are all consistent with one another. The differencesbetween them are not clear-cut, and the arbitrary names by which they are called do not countfor perception. If you know what can be done with a graspable detached object, what it can beused for, you can call it whatever you please.

The theory of affordances rescues us from the philosophical muddle of assuming fixed classesof objects, each defined by its common features and then given a name. As Ludwig Wittgensteinknew, you cannot specify the necessary and sufficient features of the class of things to which aname is given. They have only a "family resemblance." But this does not mean you cannot learnhow to use things and perceive their uses. You do not have to classify and label things in orderto perceive what they afford.

THE INFORMATION FOii VlSllAl PFRC'l'PTION

than it is to perceive all the variables separately. It is never necessary to distingui�h �lthe features of an object and, in fact, it would be impossible to do so. Perception 1seconomical. "Those features of a thing are noticed which distinguish it from otherthings that it is not-but not all the features that distinguish it from everything that itis not" (Gibson, 1966b, p. 286).

OTHER PERSONS AND ANIMALS

The richest and most elaborate alfordances of the environment are provided by otheranimals and for us, other people. These are, of course, detached objects with topo­logically clo�ed surfaces, but they change the shape of their surfaces while yet retainingthe same fundamental shape. They move from place to place, changing the postures oftheir bodies, ingesting and emitting certain substances, and doing all this sponta­neously, initiating their own movements, which is to say that their movements areanimate. These bodies are subject to the laws of mechanics and yet not subject to theJaws of mecha nics, for they are not governed by these laws. They are so different fromordinary objects that infants learn almost immediately to distinguish them �om plan�sand nonliving things. When touched they touch back, when struck they strike back; mshort, they interact with the observer and with one another. Behavior affords behavior,and the whole subject matter of psychology and of the social sciences can be thought ofas an elaboration of thi� basic fact. Sexual behavior. nurturing behavior, fighting be­havior, cooperative behavior, economic behavior, political behavior-all depend on theperceiving of w hat another person or other persons afford, or sometimes on the mis­perceiving of it.

W hat the male affords the female is reciprocal to what the female affords the male;what the infant affords the mother is reciprocal to what the mother affords the infant;what the prey affords the predator goes along with what the predator affords the prey;what the buyer affords the seller cannot be separated from what the seller affords thebuyer, and so on. The perceiving of these mutual alfordances is enormously complex,but it is nonetheless lawful, and it is based on the pickup of the information in touch,sound, odor, taste, and ambient light. It is just as much based on stimulus informatio,nas is the simpler perception of the support that is offered by the ground under one sfeet. For other animals and other persons can only give off information about themselvesinsofar as they are tangible, audible, odorous, tastable, or visible.

The other person, the generalized other, the alter as opposed to the ego, is an ecological object with a skin, even if clothed. It is an object, although it is not merelyan object, and we do right to speak of he or she instead of it. But the other person has

THF THFORV OF AFFOIIOANC:F.S

a surface that reflects light, and the information to specify what he or she is, invites, promises, threatens, or does can be found in the light.

PLACES AND HIDING PLACES

The habitat of a given animal contains places. A place is not an object with definite boundaries but a region (Chapter 3). The different places of a habitat may have d ifferent affordances. Some are places where food is usually found and others where it is not. There are places of danger, such as the brink of a cliff and the regions where predators lurk. There are places of refuge from predators. Among these is the place where mate and young are, the home, which is usually a partial enclosure. Animals are skilled at what the psychologist calls place-learning. They can find their way to significant places.

An important kind of place, made intelligible by the ecological approach to visual perception, is a place that affords concealment, a hiding place. Note that it involves social perception and raises questions of epistemology. The concealing of oneself from other observers and the hiding of a detached object &om other observers have different kinds of motivation. As every child discovers, a good hiding place fur one's body is not necessarily a good hiding place for a treasure. A detached object can be concealed both from other observers and from the observer himself The observer's body can be concealed &om other observers but not from himself, as the last chapter emphasized. Animals as well as children hide themselves and also hide objects such as food.

One of the laws of th<: ambient optic array (Chapter 5) is that at any fixed point of observation some parts of the environment are revealed and the remaining parts are concealed. The reciprocal of this law is that the observer himself, his body considered as part of the environment, is revealed at some flxed points of observation and concealed at the remaining points. An observer can perceive not only that other observers are unhidden or hidden from him but also that he is hidden or unhidden from other observers. Surely, babies playing peek-a-boo and children playing hide-and-seek are practicing this kind of apprehension. To hide is to position one's body at a place that is concealed at the points of observation of other observers. A "good" hiding place is one that is concealed at nearly all points of observation.

All of these facts and many more depend on the principle of occluding edges at a point of observation, the law of reversible occlusion, and the facts of opaque and nonopaque substances. What w� call privacy in the design of housing, for example, is the providing of opaque enclosures. A high degree of concealment is afforded by an enclosure, and complete concealment is afforded by a complete enclosure. But note that there are peepholes and screens that permit seeing without being seen. A trans­parent sheet of glass in a window transmits both illumination and information, whereas

THE INFORMATION FOR VISUAL

a translucent sheet transmits illumination but not information. There will be more of this in Chapter 11.

Note also that a glass wall affords seeing throug h but not walking through, whereas a cloth curtain affords going through but not seeing through. Architects and designers know such facts, but they lack a theory of affordances to encompass them in a system.

SUMMARY: POSITIVE AND NEGATIVE AFFORDANCES

The foregoing examples of the affordances of the environment are enough to show how general and powerful the concept is. Substances have biochemical offerings and afford manufacture. Surfaces afford posture, locomotion, collision, manipulation, and in gen­eral behavior. Special forms of layout afford shelter and concealment. Fires afford warming and burning. Detached objects-tools, utensils, weapons-afford special types of behavior to primates and humans. The other animal and the other person provide mutual and reciprocal affordances at extremely high levels of behavioral complexity. At the highest level, when vocalization becomes speech and manufactured displays become images, pictures, and writing, the affordances of human behavior are staggering. No more of that will be considered at this stage except to point out that speech, pictures, and writing still have to be perceived.

At all these levels, we can now observe that some offerings of the environment are benefic ial and some are injurious. These are slippery terms that should only be used with great care, but if their meanings are pinned down to biological and be havioral facts the danger of confusion can be minimized. First, consider substances that afford ingestion. Some afford nutrition for a given animal, some afford poisoning, and some are neutral. As I pointed out before, these facts are quite distinct from the affording of pleasure and displeasure in eating, for the experiences do not necessarily correlate with the biological effects. Second, consider the brink of a cliff. On the one side it affords walking along, locomotion, whereas on the other it affords falling off, injury. Third, consider a detached object with a sharp edge, a knife. It affords cutting if manipulated in one manner, but it affords being cut if manipulated in another manner. Similarly, but at a different level of complexity; a middle-sized metallic object affords grasping, but if charged with current it affords electric shock. And fourth, consider the other person. The animate object can give caresses or blows, contact comfort or contact injury, reward or punishment, and it is not always easy to perceive which will be provided. Note that all these benefits and injuries, these safeties and dangers, t�ese positive and negative affordances are properties of things taken with reference to an observer but not properties of the experiences of the observer. They are not subjective values; they are not feelings of pleasure or pain added to neutral perceptions.

There has been endless debate among philosophers and psychologists as to whether values are physical or phenomenal, in the world of matter or only in the world of mind. For aHordances as distinguished from values, the debate does not apply. Aflordances are neither in the one world or the other inasmuch as the theory of two worlds is rejected. There is only one environment, although it contains many observers with limitless opportunities for them to live in it.

THE ORIGIN OF THE CONCEPT OF

AFFORDANCES: A RECENT HISTORY

The gestalt psychologists recognized that the meaning or the value of a thing seems to be perceived just as immediately as its color. The value is clear on the face of it, as we say, and thus it has a physiognomic quality in the way that the emotions of a man

.�ppear on his face. To quote from the Principles of Gestalt Psychology (Koflka, 1935),Each thing says what it is …. a fruit says 'Eat me'; water says 'Drink me'; thunder

says 'Fear me'; and woman says 'Love me' " (p. 7). These values are vivid and essential features of the experience itself. Koflka did not believe that a meaning of this sort could be explained as a pale context of memory images or an unconscious set of response tendencies. The postbox "invites" the mailing of a letter, the handle "wants to be grasped," and things "t:-11 us what to do with them" (p. 353). Hence, they have what Koflka called "demand character."

Kurt Lewin coined the term Aufforderungscharakter, which has been translated as invitation character (by J. F. Brown in 1929) and as valence (by D. K. Adams in 1931; cf Marrow, 1969, p. 56, for the history of these translations). The latter term came into general use. Valences for Lewin had corresponding vectors, which could be represented as arrows pushing the observer toward or away from the object. What explanation could he given for these valences, the characters of objects that invited or demanded behavior? No one, not even the gestalt theorists, could think of them as physical and, indeed, they do not fall within the province of ordinary physics. They must therefore be phenomenal, given the assumption of dualism. If there were twoobjects, and if the valence could not belong to the physical object, it must belong to the phenomenal object-to what Koflka called the "behavioral" object but not to the "geographical" object. The valence of an object was bestowed upon it in experience and bestowed by a need of the observer. Thus, Koflka argued that the postbox has � demand character only when the observer needs to mail a letter. He is attracted to it when he has a letter to post, not otherwise. The value of something was assumed to change as the need o f the observer changed.

The concept of affordance is derived from these concepts of valence, invitation, and demand but with a crucial difference. The affordance of something does not change

E INFORMATION FOR VISUAL PERCEPTION

as the need of the observer changes. The observer may or may not perceive or attend to the affordance, according to his needs, but the affordance, being invariant, is always there to be perceived. An alfurdance is not bestowed upon an object by a need of an observer and his act of perceiving it. The object offers what it does because it is what it is. To be sure, we define what it is in terms of ecological p hysics instead of physical physics, and it therefore possesses meaning and value to begin with. But this is meaning and value of a new sort.

For Koflka it was the phenomenal postbox that invited letter-mailing, not the physical postbox. But this duality is pernicious. I prefer to say that the real postbox (the only one) affords letter-mailing to a letter-writing human in a community with a postal system. This fact is perceived when the postbox is identified as such, and it is apprehended whether the postbox is in sight or out of sight. To feel a special attraction to it when one has a letter to mail is not surprising, but the main fact is that it is perceived as part of the environment-as an item of the neighborhood in which we live. E veryone above the age of six knows what it is fur and where the nearest one is. The perception of its affordance should therefure not be confused with the temporary special attraction it may have.

The gestalt psychologists explained the directness and immediacy of the experience of valences by postulating that the ego is an object in experience and that a "tension .. may arise between a phenomenal object and the phenomenal ego. When the object is in "a dynamic relation with the ego" said Koflka, it has a demand character. Note that the "tension," the "relation," or the "vector" must arise in the "field," that is, in the field of phenomenal experience. Although many psychologists find this theory intelli­gible, I do not. There is an easier way of explaining why the values of things seem to

Figure 8.1

The changing perspective structure of a postbox during approach by an observer.

As one reduces the distance to the object to one-third, the visual solid angle of the object increases

three times. Actually this is only a detail near the center of an outflowing optic array. (From The

Perception of the Visual World by James Jerome Gibson and used with the agreement of the reprint publisher. Greenwood Press, Inc.)

THE THEORY OF AFFORDANCES

L_�I u

I

be perceived immediately and directly. It is because the alfordances of things for an

observer are specified in stimulus in:lormation. They seem to be perceived directly

because they are perceived directly. The accepted theories of perception, to which the gestalt theorists were objecting,

implied that no experiences were direct except sensations and that sensations mediated

all other kinds of experience. Bare sensations had to be clothed with meaning. The

seeming directness of meaningful perception was therefore an embarrassment to the

orthodox theories, and the Gestaltists did right to emphasize it. They began to under­

mine the sensation-based theories. But their own explanations of why it is that a fruit

says "Eat me" and a woman says "Love me" are strained. The gestalt psychologists

objected to the accepted theories of perception, but they never managed to go beyond

them.

THE OPTICAL INFORMATION FOR

PERCEIVING AFFORDANCES

The theory of affordances is a radical departure from existing theories of value and

meaning. It begins with a new definition of what value and meaning are. The perceiving

of an affordance is not a process of perceiving a value-free physical object to which

meaning is somehow added in a way that no one has been able to agree upon; it is a

process of perceiving a value-rich ecological object. Any substance, any surface, any

layout has some affordance for benefit or injury to someone. Physics may be value­

free, but ecology is not.

The 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. The

skeptic may now be convinced that there is information in light for some properties of

a surface but not for such a property as being good to eat. The taste of a thing, he will say, is not specified in light; you can see its form and color and texture but not its

palatability; you have to taste it for that. The skeptic understands the stimulus variables

that specify the dimensions of visual sensation; he knows from psychophysics that brightness corresponds to intensity and color to wavelength of light. He may concede

the invariants of structured stimulation that specify surfaces and how they are laid out

and what they are made of. But he may boggle at invariant combinations of invariants

that specify the affordances of the environment for an observer. The skeptic familiar

with the experimental control of stimulus variables has enough trouble understanding

the invariant variables I have been proposing without being asked to accept invariants

of invariants.

—–·—�E INFO

_RMATION FOR VISUAL PERCEPTION

Nevertheless, a unique combination of invariants, a compound invariant, is just

another invariant. It is a unit, and the components do not have to be combined or

associated. Only if percepts were combinations of sensations would they have to be

associated. Even in the classical terminology, it could be argued that when a number

of stimuli are completely covariant, when they always go together, they constitute a

single "stimulus." If the visual system is capable of extracting invariants from a changing

optic array, there is no reason why it should not extract invariants that seem to us

highly complex.

The trouble with the assumption that high-order optical invariants specify high­

order alfordances is that experimenters, accustomed to working in the laboratory with

low-order stimulus variables, cannot think of a way to measure them. How can they

hope to isolate and control an invariant of optical structure so as to apply it to an

observer if they cannot quantify it? The answer comes in two parts, I think. First, they

should not hope to apply an invariant to an observer, only to make it available, for it

is not a stimulus. And, second, they do not have to quantify an invariant, to apply

numbers to it, but only to give it an exact mathematical description so that other

experimenters can make it available to their observers. The virtue of the psychophysical

experiment is simply that it is disciplined, not that it relates the psychical to the

physical by a metric formula.

An affordance, as I said, points two ways, to the environment and to the observer.

So does the information to specify an affurdance. But this does not fo the least imply

separate realms of consciousness and matter, a psychophysical dualism. It says only

that the information to specify the utilities of the environment is accompanied by

information to specify the observer himself, his body, legs, hands, and mouth. This is

only to reemphasize that exteroception is accompanied by proprioception-that to

perceive the world is to coperceive oneself. This is wholly inconsistent with dualism in

any form, either mind-matter dualism or mind-body dualism. The awareness of the

world and of one's complementary relations to the world are not separable.

The child begins, no doubt, by perceiving the affordances of things fur her, for her

own personal behavior. She walks and sits and grasps relative to her own legs and body

and hands. But she must learn to perceive the affordances of things for other observers as well as for herself. An affordance is often valid for all the animals of a species, as

when it is part of a niche. I have described the invariants that enable a child to perceive

the same solid shape at different points of observation and that likewise enable two or

more children to perceive the same shape at different points of observation. These are

the invariants that enable two children to perceive the common affordance of the solid

shape despite the different perspectives, the affordance of a toy, for example. Only

when each child perceives the values of things for others as well as for herself does she

begin to be socialized.

THE THEORY OF ,U'FORn•"'""'�

MISINFORMATION FOR AFFORDANCES

If there is information in the ambient light for the affordances of things, can there alsobe misinformation? According to the thoery being developed, if information is pickedup perception results; if misinformation is picked up misperception results. The brink of a cliff affords falling off; it is in fact dangerous and it looks dangerousto us. It seems to look dangerous to many other terrestrial animals besides ourselves,including infant animals. Experimental studies have been m ade of this fact. If a sturdysheet of plate glass is extended out over the edge it no longer affords falling and in factis not dangerous, but it may still look dangerous. The optical information to specifydepth-downward-at-an-edge is still present in the ambient light; for this reason thedevice was called a visual cliff by E. J. Gibson and R. D. Walk (1960). Hapticinformation was available to specify an adequate surface of support, but this wascontradictory to the optical information. When human infants at the crawling stage oflocomotion were tested with this apparatus, many of them would pat the glass withtheir hands but would not venture out on the surface. The babies misperceived theaffurdance of a transparent surface for support, and this result is not surprising. Similarly, an adult can misperceive the affordance of a sheet of glass by mistakinga closed glass door for an open doorway and attempting to walk t hrough it. He then crashes into the barrier and is injured. The affordance of collision was not specified bythe outflow of optical texture in the array, or it was insufficiently specified. H e mistookglass for air. The occluding edges of the doorway were specified and the empty visualsolid angle opened up symmetrically in the normal manner as he approached, so hisbehavior was properly controlled, but the imminence of collision was not noticed. Alittle dirt on the surface, or highlights, would have saved him. These two cases are instructive. In the first a surface of support was mistaken forair because the optic array specified air. In the second case a barrier was mistaken forair for the same reason. Air downward affords falling and is dangerous. Air forwardaffords passage and is safe. The mistaken perceptions led to inappropriate actions. Errors in the perception of the surface of support are serious for a terrestrialanimal. If quicksand is mistaken for sand, the perceiver is in deep trouble. If a coveredpitfall is taken for solid ground, the animal is trapped. A danger is sometimes hidden-

THINGS THAT LOOIC LIKE WHAT THEY ARE If the affordances of a thing are perceived correctly, we say that it looks like w hat it is. But we must, of course, learn to see w hat things really are-for example, that the innocent-looking leafis really a nettle or that the helpful-sounding politician is really a demagogue. And this can bevery difficult.

d th Calm water and the electric shock in the radio cabinet. In the the shark un er e h ·fi · J · I · ment poison ivy is frequently mistaken fur ivy. In t e artt c1a env1-natura environ

ronment acid can be mistaken for water. k l"k h tA '.ldcat may be hard to distinguish from a cat, and a thief may loo_ 1 e an on.es WWI h K Ilka asserted that "each thing says what it is," he faded to mentionperson. en o . . that it may lie. More exactly, a thing may not look like what it is.

Nevertheless, however true all this may be, the basic affordances of t�e environ­ment are perceivable and are usually perceivable directly, without an excessive amount f l · g The basic properties of the environment that make an affordance are o eamm ·

d · If· ·fi d5 ecified in the structure of ambient light, and hence the affor ance 1tse . is spec1 e i� ambient light. Moreover, an invariant variable that is commensurate with th� bod� of the observer himself is more easily picked up than one not commensurate with his body.

SUMMARY

The medium substances, surfaces, objects, places, and other animals have affordances fo · '· al They offer benefit or inJ'ury, life or death. This is why they need to r a given amm be perceived.

f, f h . I togetherThe possibilities of the environment and the way of Ii e o t e amma go · bl The envi·ronment constrains what the animal can do, and the concept of msepara Y·

· J It t h · h · cology re11ects this fact Within limits, the human amma can a er e a me e m e u · . . affordances of the environment but is still the creature of his or her_ situation.' There is information in stimulation for the physical pro�rhes of thm_gs, andpresumably there is information for the environmental properties. The doctrme th�t says we must distinguish among the variables of things bef�re we can learn the1�· · t• ab) Affordances are properties taken with reference to the ob meanings 1s ques ,on e. server. They are neither physical nor phenomenal. . . _The hypothesis of information in ambient light to specify affordances 1s the cul mination of ecological optics. The notion of invariants that are related at one extreme to the motives and needs of an observer and at the other extreme to the substances and surfaces of a world provides a new approach to psychology.