Optical illusionIn visual perception, an optical illusion (also called a visual illusion) is an illusion caused by the visual system and characterized by a visual percept that arguably appears to differ from reality. Illusions come in a wide variety; their categorization is difficult because the underlying cause is often not clear but a classification proposed by Richard Gregory is useful as an orientation. According to that, there are three main classes: physical, physiological, and cognitive illusions, and in each class there are four kinds: Ambiguities, distortions, paradoxes, and fictions.
Gestalt therapyGestalt therapy is a form of psychotherapy that emphasizes personal responsibility and focuses on the individual's experience in the present moment, the therapist–client relationship, the environmental and social contexts of a person's life, and the self-regulating adjustments people make as a result of their overall situation. It was developed by Fritz Perls, Laura Perls and Paul Goodman in the 1940s and 1950s, and was first described in the 1951 book Gestalt Therapy.
AdditionAddition (usually signified by the plus symbol ) is one of the four basic operations of arithmetic, the other three being subtraction, multiplication and division. The addition of two whole numbers results in the total amount or sum of those values combined. The example in the adjacent image shows two columns of three apples and two apples each, totaling at five apples. This observation is equivalent to the mathematical expression "3 + 2 = 5" (that is, "3 plus 2 is equal to 5").
