June 1971
On the Distinction between Objects and Substances
J. J. Gibson, Cornell University
The World Wide Web distribution of James Gibson’s “Purple Perils” is for scholarly use with the understanding that Gibson did not intend them for publication. References to these essays must cite them explicitly as unpublished manuscripts. Copies may be circulated if this statement is included on each copy.
An ecological approach to visual perception needs to discover the optical basis for the discrimination between objects, on the one hand, and what I will call substances, on the other. “A stone” is an object but “stone” is a substance. In the book of photographs called simply Textures by Brodatz, nearly all the pictures are of substances, not objects. This comes about, I think, because they represent more or less flat surfaces, of various kinds, but not one surface in front of another, that is, there are not visible edges. In a looser terminology, there are “textures” in the photographs but not “forms.” The observer can say what is there (grass, fur, wood, cloth, water, sand, marble, etc.) but he cannot say whether there is or is not a discrete unit of what is there.
The term “object” is used so broadly in epistemology and psychology that it is a source of confusion. Let us try to delimit the meaning of the term. Perhaps it can be specified by the theory of surface layout which underlies ecological optics.
The difference between objects and substances is implied by the distinction in linguistics between two kinds of nouns, “count” nouns and “mass” nouns. The former can be pluralized; the latter cannot. The word “book” refers to something that can be enumerated as “one” or “more than one” whereas the word “paper” does not. One says “a piece of paper” but not “a piece of book.” In general, it would seem that objects can be counted but that the operation of counting does not apply to substances.
It is immediately evident that the distinction between a set of denumerable objects and a substance is not clear-cut. Intermediate between a collection of rocks on the beach and the grains of sand there may be stones and pebbles of continually decreasing size. But this does not imply that the distinction between rocks and sand is not valid, for when the child plays with rocks on the beach he acts differently from when he plays with sand. The two afford different kinds of manipulation: a rock can be dropped but sand can be poured, and rocks can be arranged but sand can be shaped. These affordances, moreover, can be seen, and I therefore assume that when the child learns to apply nouns to the entities of his environment he has already distinguished objects from substances in immediate perception. His use of nouns thus depends on his perceptions, not the other way around as seem linguists have implied.
An environmental surface, I have suggested, is the interface between a substance and the medium in which an observer gets about. A surface has a characteristic texture and a characteristic surface-color (reflectance) both of which depend on the kind of substance composing it. A surface also has certain properties and features constituting what I call its “layout,” and this can vary independently of its texture and color. The variables of layout are flatness or curvature, and slant relative to gravity; the features of layout are dihedral angles with their edges, and curved convexities or concavities. The variables and the features combine to yield an enormous variety of surface shapes. (These are the shapes of solid geometry, not the “forms” of plane geometry.) The texture and the color that characterize a surface are persistent and do not change much in the course of environmental events but the shape of a surface is persistent only to the extent that the substance is solid and thus resists deformation, or that it is undisturbed. The surface is deformed when the substance is viscous or visco-elastic, and is changed rapidly and completely when the substance is liquid, or granular. The non-rigidity of a substance is made evident by the change of shape that occurs when it is acted upon. Children are curious about and observant of such shape changes. All the above assertions come from what might be called ecological physics, not from particle physics or physical optics.
It is now possible to suggest what the difference is between a substance and an object. Both have a surface with a visible texture and color but only an object has a persisting shape. The distinctive features of a substance are its surface texture and its surface color but not its surface shape; the distinctive features of an object are its surface texture, its surface color, and its surface shape. The shape of a child’s ball is invariant; the shape of his milk is variant. It is not that a substance is “formless,” in some mysterious sense, but only that it is polymorphic. (The shape of a child’s mother consists of certain overall invariant features by which she is distinguished from other animals, other persons, and other mothers. On these constants are superposed minor changes of shape that constitute her behavior and her expressive movements. Thus the mother is an object in this terminology, but an animate object.)
Even a very young child is able to distinguish ball from milk. A ball rolls and affords grasping and dropping; milk flows and affords spilling, pouring, dabbling, and smearing, beside drinking. The shape of the ball does not change; the shape of the milk does. The general differences between objects and substances can be learned by the child in the course of noticing environmental events and causing such events by his own action.
An object, then, is a persistently and characteristically shaped surface. The bounding surface may be wholly closed (by analogy with a closed contour in two dimensions that returns upon itself) but need not be so for it to constitute an object. A blade of grass, a tree, and even a house are objects although they are not detached from the ground. A discrete object is detached and movable, like a ball, a book, an automobile, or a man. If a discrete object is within certain limits of size, it is graspable; if it is not too heavy it is also portable; if it has the right weight and size it is throwable; if it has the appropriate shape it becomes a club, or a hammer, or an axe or a spear.
We can now bring in occlusion, i.e., the fact of the projection or non-projection of surface at points of observation. Objects always occlude. The back of the object and the background are unprojected. But the observer can always walk around the object, or turn it around, and this is presumably the natural way of perceiving an object. The back of the object is always turning into the front of the object. The so-called “form” of the object, in the sense of its silhouette or its pictorial figure, is nothing other than its occluding edges, changing in time. The distinctive features of the object (the color and the texture of its surface and the set of corners, edges or curvatures of its surface that constitute its solid shape) are invariant.
The surface of the earth, on the other hand, does not occlude, and the ground is not an object. The substances of the environment do not hide anything behind them. They do not have a back side, nor is there a background. The floor is an important component of the environment of a crawling infant, his surface of gravitational support and a source of “optical support,” but it is not an object.
Summary. Prior to studying perception it is well to ask what there is to be perceived. The traditional answer is “objects,” accompanied by a recitation of dogmas about “space.” The above discussion of surfaces, substances, and objects is at least an improvement on the traditional answer. It is not a complete answer, however, for the environment has much more in it to be perceived than this. There are many non-objects in the world to be considered. There are apertures, holes, windows, and entrances. There are falling-off places or cliffs. There are interspaces and rooms. There are events. There are what I have called “vistas,” and chains of vistas with transitions between them. There is above all the sky, and the horizon of the earth at which the sky begins, and mysterious events in the sky, like rainbows, sunsets, and beams of light. And, of course, there is the social environment and the symbolic environment.
The general problem of how we perceive substances and objects (as defined) and all these other interrelated features of the integrated environment is a problem for the experimental psychologist. To solve it he has to bring it into the laboratory and devise experimental methods for testing hypotheses. But he has entirely misconceived what there is to be perceived, and so his methods and experiments have been mostly irrelevant. He has been diverted into studying lines on paper, for example, and misled by the fascinations of the tachistoscope. He will have to make a fresh start.