Sometimes towers which had looked round from a distance appeared square from close up; and enormous statues standing on their pediments did not seem large when observed from the ground.-Descartes, Meditation VI, CSM:II, 53.There is a class of phenomena that suggests strongly that perception is, in some sense to be explained, ambiguous in what it tells us about the world. In the cases at issue, the perceptual system is capable of responding to a single stimulus -say, as manifested in the ways in which subjects sort that stimulus -in different ways. Indeed, in many cases, subjects can be made to switch at will between these different modes of response. This paper is about that ambiguity, and about how it should be characterized and accounted for within a general theory of perception.
The Puzzle Cases: The Ambiguity of PerceptionLet me begin, then, by bringing to mind some (I hope) familiar cases in which our perceptual systems generate multiple reactions to one stimulus. 1 (As usual in such discussions, I'll be focusing below mostly on visual perception; but I suspect that much of what I say can be generalized to at least some other perceptual modalities as well.)A first classic example is that of the non-uniformly illuminated but uniformly painted wall (e.g., as in figure 1). Consider two contiguous and samesized, same-shaped regions of the wall in figure 1 that are each (roughly) illuminated uniformly, but such that there is a difference in the illumination between the two -one is in shadow and one is in direct sunlight. Here is a fact. Subjects perceiving this stimulus condition can indicate about the differently illuminated regions (say, by their sorting behavior) that they are, in one way, alike in their color appearance and that they are, in another way, not alike in their color appearance. It seems that visual systems can pick up both the constancy/similarity or the inconstancy/dissimilarity between the regions, and subjects can respond (say, in the ways that they sort, report, or make matches) to either one. Moreover, famously, ordinary subjects can 1 be made to switch between these different modes of response simply by manipulating the task instructions (Arend and Reeves, 1986;Blackwell and Buchsbaum, 1988;Valberg and Lange-Malecki, 1990;Arend et al., 1991;Troost and deWeert, 1991;Cornelissen and Brenner, 1995;Bäuml, 1999). These familiar results give us powerful reason for believing that, in the kind of case at issue, perception is representing distinct things about the single distal stimulus. Assuming (standardly, though not uncontroversially) that perceptual states have representational contents, we can put this point by saying that the perceptual states caused by such stimuli have (at least) two different contents -one representing the aspect of similarity driving one kind of subject response, and one representing the aspect of dissimilarity driving the other kind of subject response. Here is another familiar example of the same sort of phenomenon. Viewing two telephone poles at different distances, s...