need in life, not just pictorial depth perception. We need to see all the way around at
a given point of observation and to take different points of observation. The crux of the
matter is whether or not natural vision is compounded of units like the snapshot. I
very much doubt that vision is simplest when the experimenter tries to make the eye
work as if it were a photographic camera, even the kind that takes pictures in rapid
succession.

Looking around and getting around do not fit into the standard idea of what visual
perception is. But note that if an animal has eyes at all it swivels its head around and
it goes from place to place. The single, frozen field of view provides only impoverished
information about the world. The visual system did not evolve for this. The evidence
suggests that visual awareness is in fact panoramic and does in fact persist during long
acts of locomotion.

Part I of this book is about the environment to be perceived. Part II is about the
information for perception. Part III is about the activity of perception. Finally, Part IV
is about pictures and the special kinds of awareness that go with looking at them.
Picture vision comes last because it can be understood only after we are clear about
ambient vision and ambulatory vision.

First, the environment must be described, since what there is to be perceived has
to be stipulated before one can even talk about perceiving it. This is not the world of
physics but the world at the level of ecology. Second, the information available for
perception in an illuminated medium must be described. This is not just light for
stimulating receptors but the information in the light that can activate the system.
Ecological optics is required instead of classical optics. Third (and only here do we
come to what is called psychology proper), the process of perception must be described.
This is not the processing of sensory inputs, however, but the extracting of invariants
from the stimulus flux. The old idea that sensory inputs are converted into perceptions
by operations of the mind is rejected. A radically new way of thinking about perception
is proposed.

The ecological approach to perception was adopted in my book The Senses Con-
sidered as Perceptual Systems, which came out in 1966. Actually, it is a new approach
to the whole field of psychology, for it involves rejecting the stimulus-response formula.
This notion, borrowed from the so-called hard science of physiology, helped to get rid
of the doctrine of the soul in psychology, but it never really worked. Neither mentalism
on the one hand nor conditioned-response behaviorism on the other is good enough.
What psychology needs is the kind of thinking that is beginning to be attempted in
what is loosely called systems theory.

Environmentalism is a powerful movement nowadays, but in psychology it has
generated more enthusiasm than discipline. There is no central core of theoretical
concepts on which to base it. The right conceptual level has not yet been found. This
book makes an effort to find the right level. A few psychologists, such as E. Brunswik

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