BARNARD@SRI-STRIPE.ARPA (Stephen Barnard) (07/19/86)
Eyal Mozes thinks that direct perception is right on, and that internal representations either don't exist or aren't important. I think direct perception is a vague and suspiciously mystical doctrine that has no logical or physical justification. Barnard: >>Consider what happens when we look at a realistic >>painting. We can, at one level, see it as a painting, or we can see >>it as a scene with no objective existence whatsoever. How could this >>perception possibly be interpreted as anything but an internal >>representation? Mozes: >Sorry, I can't follow your argument. Of course, a realistic painting is >a representation; but it is not an INTERNAL representation. Gibson's >books do contain long discussions of paintings; but he specifically >distinguishes between looking at a painting (in which case you are >perceiving a representation of the object) and directly perceiving the >object itself. Barnard's reply: Look, the painting is a representation, but we don't perceive it AS a representation --- we perceive it as a scene. The scene has NO OBJECTIVE EXISTENCE; therefore, we cannot perceive it DIRECTLY. It exists only in our imaginations, presumably as internal representations. (How else?) If the painter was skillful, the representations in our imagination match his intention. To counter this argument, you must tell me how one can "directly" perceive something that doesn't exist. Good luck. On the other hand, it is quite possible to merely represent something that doesn't exist. Barnard: >>Gibson emphasized the richness of the visual stimulus, >>arguing that much more information was available from it than was >>generally realized. But to go from this observation to the conclusion >>that the stimulus is in all cases sufficient for perception is clearly >>not justified. Mozes: >Gibson did not deny that there are SOME cases (for example, many >situations created in laboratories) in which the stimulus is >impoverished. His point was that these cases are the exception, rather >than the rule. Even if we agree that in those exceptional cases there >is some inference from background knowledge, this doesn't justify >concluding that in the normal cases, where the stimuli do uniquely >specify the external object, inference also goes on. To the contrary, ambiguous visual stimuli are not rare exceptions --- the visual stimulus is ambiguous in virtually EVERY CASE. Gibson was fond of stereo and optic flow as modes of perception that can disambiguate static, monocular stimuli (which are clearly ambiguous). But he simply did not realize that such modalities are themselves ambiguous. For example, I am not aware of Gibson discussing the aperture problem, which describes ambiguity in optic flow. Similarly, depth from stereo is unique once the image-to-image correspondence is achieved, but, as we know from years of research on computational stereo, solving the correspondence problem is not easy, primarly due to the problem of resolving ambiguous matches. Similar problems occur for every mode of visual perception. Gibson's hypothesis that the information for perception exists completely in the stimulus is false, and the entire theory of direct perception falls apart as a consequence. -------