HARMONY, in philosophy, de notes the relationship between body and mind as conceived by Leibnitz. According to the interaction theory body and mind act upon each other, so that the body does what the mind wants done, and the mind is affected by the condition of the body. Cartesian dualism excluded the possibility of such interaction, by its conception of an absolute difference and opposition between mental and material substances. To account for the appearance of such interaction some of the Cartesians introduced the theory of "occasionalism," namely, that the mind does not influence the body nor vice versa, but that on every occasion that anything happens to the one, God produces a certain change in the other. This implied God's constant interference with everything. Leibnitz suggested that body and mind have been harmonized by God once for all, so that their changes correspond or synchronize with out either influencing the other, and without needing God's inces sant intervention. He compares the relation between body and
mind to that between two clocks which have been synchronized once for all, and so keep step without mutual interaction and with out the constant intervention of anybody. This is the theory of pre-established harmony, which Leibnitz applied to his system of monads. For Leibnitz reality consists of monads or spirits of different degrees of development. Their active side is their spirit ual side, and matter is simply the appearance of their passivity or deficiency. As their activity is entirely immanent, the monads do not interact (they "have no windows"), but there is a pre-estab lished harmony between them inasmuch as they all mirror the same universe, though each "from its own point of view." See LEIBNITZ.