March 1974
A Further Note on the Perceiving of Hidden and Unhidden Surfaces
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 References to these essays must cite them explicitly as unpublished manuscripts.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.
There are still more facts or laws of ecological optics that deal with hidden and unhidden surfaces, and the interchanges between them, beyond those listed in the memo of February, 1974. They also have further implications.
1. To every point of observation in the medium, there corresponds a unique optic array. This is because a surface layout at one place in the world is not duplicated at any other; one “scene” is not repeated elsewhere. This law holds at the ecological but not at the atomic level.
2. To every moving point of observation in a medium, there corresponds a uniquely changing optic array, the change being specific to the path of locomotion.
Now consider objects and enclosures, an object such as a table and an enclosure such as a room.
3. To every object or enclosure in the world, there corresponds a unique family of optic arrays at all the points of observation to which the object or enclosure is projected. We express this fact very loosely when we say that there is a family of “perspective appearances” for every object or enclosure.
The family of optic arrays can be registered in sequence by any individual observer who moves around the table or in the room; the family can be registered simultaneously by a crowd of different observers who stand around the table or in the room. (I am assuming each observer can look around at his point of observation). It follows, since all can move around, that all observers can perceive the same table, and the same room. It is only true that no observer can have the same perspective at the same time as another observer. This law is the basis of consensus.
4. To perceive an object means to perceive its far side as well as its near side, i.e., what it looks like if one walks around it (or if it is turned around). That is, to perceive something is to be aware of it from other points of observation than the stationary point being occupied. I suggest that this is entailed in perceiving.
This is what I meant when I wrote of visualizing as a kind of “apprehending without a fixed point of observation” in Leonardo (1974).
5. An observer who is aware of something from other points of observation than the one now occupied is able to take the point of view of another observer or, as we say, “see” from his point of view. If he can do so, he should be able to describe an object as it appears to another person in the room, since he can perceive that the other person is facing the “far” side of the object, or the “left” side (or whatever). Or he should be able to recognize a photograph of the object taken from the other person’s point of view. This is the experiment carried out by Piaget and Inhelder with children in The Child’s Conception of Space.
6. There is said to be egocentric perception as well as behavior. The egocentric person is said to be the one who sees the world only from his own point of view. But this, it should now be clear, is literally impossible, since the point moves, the structure of the optic array keeps changing, and the appearances fluctuate. If the world can only be perceived from a moving point of observation it is, of necessity, perceived from “other” points of view, which includes those of other persons. In short, there is no such thing as egocentric perception (there is only egocentric proprioception). The evidence does not suggest that the earliest visual experiences of the child are those of the perspective projections of objects and places; more likely the first percepts are based on what may be called “formless invariants”.
7. In short, the assertion that “I can see things from your point of view” or “I can put myself in your position” has an exact meaning in terms of ecological optics and is not merely a figure of speech. It means I can perceive surfaces that are revealed at your point of view but concealed at mine. It means I can see one thing behind another. And it implies that we both see the same world.
8. The fact that animals and men can be oriented to the environment behind the surfaces that are projected to the momentary point of view, the “hidden” environment, or to the geographical environment (Ryan and Ryan) becomes intelligible in the light of the above facts. The homing of animals and the migration of birds can be explained in this way instead of by the theory of the development of the concept of “object permanence”.