Visual illusions provide evidence of perceptual processes, especially the use of knowledge about objects and the working rules for seeing. The classification of phenomena is necessary for any science because it facilitates induction and deduction. The phenomena of illusions are no exception. It is hoped that the tentative classification presented here may stimulate further research and raise new questions by revealing gaps in the conceptual structure of how perception proceeds. Finally, a speculation is offered for the functional use of sensations, the qualia of consciousness.