Although perceptual illusions have been studied since the middle years of the 19th century there is no generally acceptable explanation of specific effects or of illusions in general. The central thesis proposed here is that illusions might be more fruitfully approached in terms of the principles of perception that they reveal and that apply also to normal, veridical perception. Recent experiments involving the conventional form of the Müller-Lyer figure presented progressively in a narrow aperture and a much simplified collinear form devised for presentation over space or time in both the visual and auditory modes suggested two such principles, that of whole-part determination of perceived size and that of space- time reciprocity. The first refers to the determination of the size of intrinsic parts by the whole figure or object and the second to the modulation of extent by time and vice versa. The broader implications of these principles are discussed.