 In pictures B and D the eye recognizes disparate shapes as "belonging" to a single shape, in C a complete three-dimensional shape is seen, where in actuality no such thing is drawn.Reification can be explained by progress in the study of illusory contours, which are treated by the visual system as "real" contours.Multistability (or multistable perception) is the tendency of ambiguous perceptual experiences to pop back and forth unstably between two or more alternative interpretations.