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            We’ve been discussing illusions as if they were abnormalities of vision, but how can we call something that is a universal and predictable experience of all human beings abnormal? When I perceive two lines slanted towards each other as going off into the distance, when they are in fact on the same plane, I am said to be manifesting an illusion. Whereas, if I consider two people, of the equal heights, to be the same height when one is standing up close and the other in the distance, my perception is not said to be illusory despite the fact that the latter projects a smaller retinal image. These instances force me to wonder which condition is the illusion and which percept is normal? Since both classes of percept are manifested by everyone, that is everyone perceives them in this way, isn’t it reasonable to classify both instances as examples of normal perception?

            Illusions are not something abnormal that cloud the “normal” perception of the real world. I would even go so far as to suggest that the perception of illusory distortions constitutes a clearly definable aspect of normal perception. Basically, any account of normal perception should also account for the perception of illusions, which should be analyzed as a part of the normal perceptual process and may serve as important tools in the study of visual perception.