“SUBSTANCE (SUBSTANTIA/SUBSTANCE). A term deriving from Aristotle to refer to the subjects of predication and the objects of scientific inquiry. It became a key term of metaphysics because substances are the fundamental entities of which the universe is constituted. In René Descartes’s philosophy, there are three distinct kinds of substance: God, matter, and minds. In the pantheistic system of Spinoza, on the other hand, there is only one substance, which he refers to as “God-or-Nature.”
Leibniz’s considered view is that there are two fundamentally different kinds of substance: God, who is a pure spirit, and created substances, all of which have bodies. All substances must, according to Leibniz, be capable of action. Only God is pure activity, that is, lacking entirely in passivity; all creatures have some activity and are in varying degrees passive. There is, for Leibniz, a hierarchy of created substances, ranging from creatures close to God, such as angels, to animals that have senses but lack reason, to even more basic corpo-real substances. Humans—capable of reason and therefore made in the image of God—are above animals but lower than angels.
A substance must, according to Leibniz, be a real unity. At one time he seems to have held the view that the unity of corporeal substances was underwritten by their substantial forms. But his later view seems to have been that every substance must be some kind of living thing, with something like perception and something like appetition. He later referred to his simple substances as monads. However, Leibniz seemed also to want to admit complex substances as more than an aggregate of simple substances. He did this by saying that, although there is nothing more to a complex substance than its constituent monads, its unity arises because one of these is the dominant monad.
Leibniz’s theory of substance is the linchpin of his metaphysics.
Each substance, according to Leibniz, is quite unique. He thought that the complete concept of each individual substance contained within itself everything that is true of it. Correspondingly, the nature or essence of each substance was such as to give rise spontaneously to all its phenomena. No substance except God can act on any other substance, nor can it be acted upon by any other substance. The appearance of interaction between substances is to be explained in terms of a preestablished harmony that God has foreordained from the beginning of time.”