Monadology (from Gr. μονάς, unity, and λόγος, discourse) is the term applied to the doctrine or science of Monads, which was filly developed by the German philosopher Leibnitz. " He conceived the whole universe, bodies as well as minds, to be made up of monads, that is, simple substances; each of which is, by the Creator, in the beginning of its existence, endowed with certain active and perceptive powers. A monad, therefore, is an active substance, simple, without parts or figure, which has within itself the power to produce all the changes it undergoes from the beginning of its existence to eternity. The changes which the monad undergoes, of what kind soever, though they may seem to us the effect of causes operating from without, are oily the gradual and successive evolutions of its own internal powers, which would have produced all the same changes and motions although there had been no other being in the universe" (Reid, Intell. Powers, essay 2, chapter 15). "Monadology," says Cousin, " rests upon this axiom: every substance is at the same time a cause, and, every substance being a cause, has therefore in itself the principle of its own development; such is the monad — it is a simple force. Each monad has relation to all others; it corresponds with the plan of the universe; it is the universe abridged; it is, as Leibnitz says, a living mirror which reflects the entire universe under its own point of view. But every monad being simple, there is no immediate action of one monad upon another; there is, however, a natural relation of their respective development, which makes their apparent communication; this natural relation, this harmony, which has its reason in the wisdom of the supreme Director, is pre-established harmony" (Hist. of Mod. Philos.