The Ecological Approach to Visual Perception

Classic Edition

James J. Gibson

• Publisher: Psychology Press
• Year: 2014
• ISBN:
  9781317579373
• Edition: 1
• Topic: Psychology
• Subtopic: History & Theory in Psychology
• Index: Psychology

This book, first published in 1979, is about how we see: the environment around us (its surfaces, their layout, and their colors and textures); where we are in the environment; whether or not we are moving and, if we are, where we are going; what things are good for; how to do things (to thread a needle or drive an automobile); or why things look as they do.

The basic assumption is that vision depends on the eye which is connected to the brain. The author suggests that natural vision depends on the eyes in the head on a body supported by the ground, the brain being only the central organ of a complete visual system. When no constraints are put on the visual system, people look around, walk up to something interesting and move around it so as to see it from all sides, and go from one vista to another. That is natural vision -- and what this book is about.

Part I The Environment to be Perceived

1 The Animal and The Environment

DOI: 10.4324/9781315740218-3

In this book, environment will refer to the surroundings of those organisms that perceive and behave, that is to say, animals. The environment of plants, organisms that lack sense organs and muscles, is not relevant in the study of perception and behavior. We shall treat the vegetation of the world as animals do, as if it were lumped together with the inorganic minerals of the world, with the physical, chemical, and geological environment. Plants in general are not animate; they do not move about, they do not behave, they lack a nervous system, and they do not have sensations. In these respects they are like the objects of physics, chemistry, and geology.

The world can be described at different levels, and one can choose which level to begin with. Biology begins with the division between the nonliving and the living. But psychology begins with the division between the inanimate and the animate, and this is where we choose to begin. The animals themselves can be divided in different ways. Zoology classifies them by heredity and anatomy, by phylum, class, order, genus, and species, but psychology can classify them by their way of life, as predatory or preyed upon, terrestrial or aquatic, crawling or walking, flying or nonflying, and arboreal or ground-living. We are more interested in ways of life than in heredity.

The environment consists of the surroundings of animals. Let us observe that in one sense the surroundings of a single animal are the same as the surroundings of all animals but that in another sense the surroundings of a single animal are different from those of any other animal. These two senses of the term can be troublesome and may cause confusion. The apparent contradiction can be resolved, but let us defer the problem until later. (The solution lies in the fact that animals are mobile.) For the present it is enough to note that the surroundings of any animal include other animals as well as the plants and the nonliving things. The former are just as much parts of its environment as the inanimate parts. For any animal needs to distinguish not only the substances and objects of its material environment but also the other animals and the differences between them. It cannot afford to confuse prey with predator, own-species with another species, or male with female.

The Mutuality of Animal and Environment

The fact is worth remembering because it is often neglected that the words animal and environment make an inseparable pair. Each term implies the other. No animal could exist without an environment surrounding it. Equally, although not so obvious, an environment implies an animal (or at least an organism) to be surrounded. This means that the surface of the earth, millions of years ago before life developed on it, was not an environment, properly speaking. The earth was a physical reality, a part of the universe, and the subject matter of geology. It was a potential environment, prerequisite to the evolution of life on this planet. We might agree to call it a world, but it was not an environment.

The mutuality of animal and environment is not implied by physics and the physical sciences. The basic concepts of space, time, matter, and energy do not lead naturally to the organism-environment concept or to the concept of a species and its habitat. Instead, they seem to lead to the idea of an animal as an extremely complex object of the physical world. The animal is thought of as a highly organized part of the physical world but still a part and still an object. This way of thinking neglects the fact that the animal-object is surrounded in a special way, that an environment is ambient for a living object in a different way from the way that a set of objects is ambient for a physical object. The term physical environment is, therefore, apt to get us mixed up, and it will usually be avoided in this book.

Every animal is, in some degree at least, a perceiver and a behaver. It is sentient and animate, to use old-fashioned terms. It is a perceiver of the environment and a behaver in the environment. But this is not to say that it perceives the world of physics and behaves in the space and time of physics.

The Difference Between the Animal Environment and the Physical World

The world of physics encompasses everything from atoms through terrestrial objects to galaxies. These things exist at different levels of size that go to almost unimaginable extremes. The physical world of atoms and their ultimate particles is measured at the level of millionths of a millimeter and less. The astronomical world of stars and galaxies is measured at the level of light-years and more. Neither of these extremes is an environment. The size-level at which the environment exists is the intermediate one that is measured in millimeters and meters. The ordinary familiar things of the earth are of this size—actually a narrow band of sizes relative to the far extremes. The sizes of animals, similarly, are limited to the intermediate terrestrial scale. The size of the smallest animal is an appreciable fraction of a millimeter, and that of the largest is only a few meters.

The masses of animals, likewise, are measured within the range of milligrams to kilograms, not at the extremes of the scale, and for good physiological reasons. A cell must have a minimum of substances in order to permit biochemical reactions; living animals cannot exceed a maximum mass of cells if they are all to be nourished and if they are to be mobile. In short, the sizes and masses of things in the environment are comparable with those of the animals.

Units of the Environment

Physical reality has structure at all levels of metric size from atoms to galaxies. Within the intermediate band of terrestrial sizes, the environment of animals and men is itself structured at various levels of size. At the level of kilometers, the earth is shaped by mountains and hills. At the level of meters, it is formed by boulders and cliffs and canyons, and also by trees. It is still more finely structured at the level of millimeters by pebbles and crystals and particles of soil, and also by leaves and grass blades and plant cells. All these things are structural units of the terrestrial environment, what we loosely call the forms or shapes of our familiar world.

Now, with respect to these units, an essential point of theory must be emphasized. The smaller units are embedded in the larger units by what I will call nesting. For example, canyons are nested within mountains; trees are nested within canyons; leaves are nested within trees; and cells are nested within leaves. There are forms within forms both up and down the scale of size. Units are nested within larger units. Things are components of other things. They would constitute a hierarchy except that this hierarchy is not categorical but full of transitions and overlaps. Hence, for the terrestrial environment, there is no special proper unit in terms of which it can be analyzed once and for all. There are no atomic units of the world considered as an environment. Instead, there are subordinate and superordinate units. The unit you choose for describing the environment depends on the level of the environment you choose to describe.

The size-levels of the world emphasized by modern physics, the atomic and the cosmic, are inappropriate for the psychologist. We are concerned here with things at the ecological level, with the habitat of animals and men, because we all behave with respect to things we can look at and feel, or smell and taste, and events we can listen to. The sense organs of animals, the perceptual systems (Gibson, 1966b), are not capable of detecting atoms or galaxies. Within their limits, however, these perceptual systems are still capable of detecting a certain range of things and events. One can see a mountain if it is far enough away and a grain of sand if it is close enough. That fact is sufficiently wonderful in itself to deserve study, and it is one of the facts that this book will try to explain.

The explanation of how we human observers, at least some of us, can visualize an atom or a galaxy even if we cannot see one will not be attempted at this stage of the inquiry. It is not so much a problem of perception as it is of thinking, and there will be more about this later. We must first consider how we can perceive the environment—how we apprehend the same things that our human ancestors did before they learned about atoms and galaxies. We are concerned with direct perception, not so much with the indirect perception got by using microscopes and telescopes or by photographs and pictures, and still less with the kind of apprehension got by speech and writing. These higher-order modes of apprehension will only be considered in Part IV of this book, at the end.

Units of the Ground Surface

The literal basis of the terrestrial environment is the ground, the underlying surface of support that tends to be on the average flat—that is to say, a plane—and also level, or perpendicular to gravity. And the ground itself is structured at various levels of metric size, these units being nested within one another. The fact to be noted now, since it is important for the theory of perspective in Part II, is that these units tend to be repeated over the whole surface of the earth. Grains of sand tend to be of the same size everywhere, and so do pebbles and rocks. Blades of grass are all more or less similar to one another, and so are clumps of grass and bushes. These natural units are not, of course, perfectly uniform like the man-made tiles of a pavement. Nevertheless, even if their repetition is not metrically regular, it is stochastically regular, that is to say, regular in a probabilistic way. In short, the component units of the ground do not get smaller as one goes north, for instance. They tend to be evenly spaced; and if they are scattered, they tend to be evenly scattered.

The Time Scale of the Environment: Events

Another difference between the environment to be described and the world of physics is in the temporal scale of the process and events we choose to consider. The duration of processes at the level of the universe may be measured in millions of years, and the duration of processes at the level of the atom may be measured in millionths of a second. But the duration of processes in the environment is measured only in years and seconds. The various life spans of the animals themselves fall within this range. The changes that are perceived, those on which acts of behavior depend, are neither extremely slow nor extremely rapid. Human observers cannot perceive the erosion of a mountain, but they can detect the fall of a rock. They can notice the displacement of a chair in a room but not the shift of an electron in an atom.

Figure 1.1 The structure of the terrestrial earth as seen from above.

In this aerial photograph only the large-scale features of the terrain are shown. (Photo by Grant Heilman)

The same thing holds for frequencies as for durations. The very slow cycles of the world are imperceptible, and so are the very rapid cycles. But at the level of a mechanical clock, each motion of the pendulum can be seen and each click of the escapement can be heard. The rate of change, the transition, is within the limits of perceptibility.

In this book, emphasis will be placed on events, cycles, and changes at the terrestrial level of the physical world. The changes we shall study are those that occur in the environment. I shall talk about changes, events, and sequences of events but not about time as such. The flow of abstract empty time, however useful this concept may be to the physicist, has no reality for an animal. We perceive not time but processes, changes, sequences, or so I shall assume. The human awareness of clock-time, socialized time, is another matter.

Just as physical reality has structure at all levels of metric size, so it has structure at all levels of metric duration. Terrestrial processes occur at the intermediate level of duration. They are the natural units of sequential structure. And once more it is important to realize that smaller units are nested within larger units. There are events within events, as there are forms within forms, up to the yearly shift of the path of the sun across the sky and down to the breaking of a twig. And hence there are no elementary units of temporal structure. You can describe the events of the environment at various levels.

The acts of animals themselves, like the events of the environment they perceive, can be described at various levels, as subordinate and superordinate acts. And the duration of animal acts is comparable to the duration of environmental events. There are no elementary atomic responses.

The natural units of the terrestrial environment and the natural units of terrestrial events should not be confused with the metrical units of space and time. The latter are arbitrary and conventional. The former are unitary in one sense of the term, and the latter are unitary in a quite different sense. A single whole is not the same as a standard of measurement.

Permanence and Change of the Layout

Space and time will not often be referred to in this book, but a great deal will be said about permanence and change. Consider the shape of the terrestrial environment, or what may be called its layout. It will be assumed that the layout of the environment is both permanent in some respects and changing in some other respects. A living room, for example, is relatively permanent with respect to the layout of floor, walls, and ceiling, but every now and then the arrangement of the furniture in the room is changed. The shape of a growing child is relatively permanent for some features and changing for others. An observer can recognize the same room on different occasions while perceiving the change of arrangement, or the same child at different ages while noticing her growth. The permanence underlies the change.

Permanence is relative, of course; that is, it depends on whether you mean persistence over a day, a year, or a millennium. Almost nothing is forever permanent; nothing is either immutable or mutable. So it is better to speak of persistence under change. The “permanent objects” of the world, which are of so much concern to psychologists and philosophers, are actually only objects that persist for a very long time.

The abstract notion of invariance and variance in mathematics is related to what is meant by persistence and change in the environment. There are variants and invariants in any transformation, constants and variables. Some properties are conserved and others not conserved. The same words are not used by all writers (for example, Piaget, 1969), but there is a common core of meaning in all such pairs of terms. The point to be noted is that for persistence and change, for invariant and variant, each term of the pair is reciprocal to the other.

Persistence in th...