Abstract: In the general theory of illusions set forth in this article, illusions as well as normal perceptions are regarded as reflex end-effects of nervous action started by a given set of stimulus conditions and fully determined in their conscious content by existing neural connections. Essentially different environmental features or characteristics of objective reality can offer the same stimulus conditions. Illusions occur when neural connections remain what they are since their establishment in evolutional and developmental history, while the environment takes on new features or reality characteristics change from the usual to the unusualFrom the point of view of the proposed theory, an illusion of visual stereo-effect (the "kite illusion"), the so-called "illusion of Aristotle", the geometrical optical illusions and the "moon illusion" are discussed. The question of the possibility of reforming undesirable illusions, of changing the illusory into normal perception, is answered in the affirmative, and the means thereto is suggested.

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