A. J. Ayer on the Argument from Illusion

In his paper “Has Austin Refuted the Sense-Datum Theory?” A. J. Ayer contends that the argument from illusion calls attention to perspectival distortion, perceptual misidentification and elusive perceptual belief only in order to establish the possibility of perceptual error. Pointing to our occasional perceptual failures reminds us that perceptual error is always logically possible—that any particular perceptual belief to the effect that one is perceiving a physical surface could be mistaken. This in turn is thought by Ayer to show that the ordinary belief that we perceive physical surfaces requires qualification—along lines urged by sense-datum philosophers—to the effect that even in those situations properly described for non-philosophical purposes as perceptions of physical surfaces it cannot be physical surfaces that are literally present to one's senses. Ayer believes that this can be established, for any given case, by the possibility of perceptual error alone.