ABSTRACT
Visual “illusions” are intriguing examples showing that what we see is not always what the ruler measures. The great nineteenth-century physiologist Helmholtz (1867, 1896) held that illusions are due to discrepancies in the very cues that normally produce accurate perception. Once we can explain what information produces accurate perception, misperceptions simply become an aspect of how we see. It seems to me that the principle proposed by Helmholtz is still valid.
