December 1969
Transparency and Occlusion or How Bishop Berkeley Went Wrong in the First Place
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 References to these essays must cite them explicitly as unpublished manuscripts.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.
In the second paragraph of the New Theory of Vision, Berkeley wrote, “It is I think agreed by all that distance of itself, and immediately, cannot be seen. For distance being a line directed end-wise to the eye, it projects only one point in the fund of the eye, which point remains invariably the same whether the distance be longer or shorter.” This states the problem of the perception of the “third dimension,” or “depth perception,” as it has been puzzled over for some 250 years.
Now I argued in the Visual World that a kind of distance could be seen, of itself and immediately, that is, distance over the ground, or along a surface. It was specified by a gradient on the retina, not by apoint on the retina; different points at different distances thus did not “project only one point in the fund of the eye”.
I now wish to argue that Bishop Berkeley’s statement of the problem was inadequate in an even more radical way. There is yet another kind of “distance” that can be seen directly, namely transparency, or the seeing of one surface through another. In this case here is an optical superposition of one surface on the other, both objects being seen in the same direction. Both objects are, indeed, projected to the same points on the retina, if the analysis of optical projection in terms of rays and points is accepted. But in terms of a different analysis, the optical information for the perception of transparency gives promise of being formulated (e.g. Metelli).
Moreover, there is a fourth kind of distance other than the abstract and invisible “line directed end-wise to the eye.” It is optical occlusion, or one surface behind another, the occluding surface being opaque rather than transparent. Our evidence, like that of Michotte, shows that the occluded surface may be perceived immediately in certain circumstances, and we have a theory of what the information is for the perception of an opaque occluding edge.
Where Bishop Berkeley went wrong was in assuming that point – sensations were necessary for perception, that is, the basis of perception. “What we immediately and properly see,” he wrote, “are only lights and colors in sundry situations and shades, and degrees of faintness and clearness.” These are the spots and patches of color composing the visual field. He added that they “are only in the mind; nor do they suggest ought external, whether distance or magnitude, otherwise than by habitual connection, as words do things.” Perhaps it is true that there can only exist one color sensation in the direction of any given light ray. But this should not count against the fact that different objects can in fact be experienced in the same direction-from-here. Nor does it count against the possibility of optical information to specify the existence of more than one surface in the same direction.