- under unusual external conditions of perception of objects, and the sense organs of the perceiving subject function normally (for example, “optical deception”);
- with unusual (pathological, affective, narcotic) states of the physiological mechanisms of the subject; under the influence of pain, intoxicating substances or affects (especially fear, expectation, hope) consciousness forms a distorted view of the objects of reality or perceives them in an inadequate manner; in this case, the illusion should be distinguished from the hallucination that occurs in the absence of external objects.
The concept of “illusion” is used as a synonym for hopes and dreams (as a rule, unreal), consoling and facilitating the life of self-deception or the flight of creative imagination (always referring to visual perception, since the very term “illusion” is based on the Latin word “lucis”). From the philosophical point of view, the desire for an illusory perception of reality is an inalienable feature of human consciousness, seeking in it a rest from “harsh reality”, hope or pleasure. So, the fear of death gives rise to the idea of the afterlife, the boredom of everyday life awakens dreams of a miracle or a “beautiful life,” the flight of creative imagination creates works of art that bring pleasure. The German philosopher K. Lange explained the origin of art precisely by the need for illusions (Lange if, Das Wesen der Kunst, 1911).
In the second half of the 20th century, thanks to the works of O. Huxley (“The Gate of Knowledge”) and K. Castaneda (“The Teaching of Don Juan: The Path of Knowledge of the Yaki Indians”), a peculiar fashion arose for illusory perception of reality that arose under the influence of the use of narcotic substances. Illusions are one of the ways to look into the caches of the human unconscious because their nutritional soil is hidden desires and unrealized hopes. Human illusions are not merely the result of a distorted perception: they are usually superimposed on the content of memory and the spontaneous activity of consciousness.