In his development of his description of the monad, Leibnitz hits upon two catch-phrases, one of which presents his doctrine of the physical isolation of the monad, the other presents the doctrine of its ideal psychical unity. These phrases are: “the monads are windowless” and “the monads mirror the universe.” By “windowless” Leibnitz means that each monad is “like a separate world, self-sufficient, independent of every other creature.” “Having no windows by which anything can enter or depart,” the monad can perceive only its own states. Whatever happens to it comes from itself alone as a purely internal principle. The monad’s development is self-development and not the result of external changes. Nevertheless the monad is a “mirror of the universe.” In this psychical qualification of the nature of the monad, its physical isolation vanishes and the way is open for a unity of monads, which would have otherwise seemed to be physically hopelessly sundered. How is it possible for each of the numberless monads, all so different, to “mirror the universe”? The answer is found in their psychical power of representation. =The Two Forms of Leibnitz’s Conception of the Unity of the Substances.= The principle of unity among the monads is called by Leibnitz a _preëstablished harmony_. He presented this principle of harmony in two ways. In part the harmony comes out of their constitution, as he conceived it to be. In part Leibnitz artificially superimposed it upon the monads for theological reasons. In either case it is preëstablished. =The Intrinsic Unity of the Monads――The Philosophical Unity.= There is a family resemblance among the monads. The lowest reproduces the universe in obscure and elementary representations. Minerals and plants are sleeping monads with entirely unconscious ideas. Animals are dreaming monads. Man is a waking monad. The highest monad is God, who reproduces the universe in clear and distinct ideas. Between God and matter there is a series of monads, graded as to the clearness of their ideas. All contain the universe by representation. All are bound together according to the principle of continuity; plants are lower animals and animals are less perfect men. Man is a monad whose conscious activity has risen to the height of self-consciousness, with the cognate power of reason. There is no inert matter; no soul-less bodies nor body-less souls. The smallest portion of dust is the habiliment of animalculæ. Nothing is dead, and nature is a gradation of monads in differing degrees of activity.