Soon after, Descartes used a thought experiment to demonstrate that the mind and body are separate substances. Each can be conceived clearly and distinctively as being completely capable of existing independently of one another. Deeply thinking about it, it's not hard to imagine your body existing without your mind. How can mind and body, two drastically different kinds of things, interact with each other? Descartes relies upon theology to answer this question. Above and beyond creating mental substance and physical substance, there is a third kind of creation. According to Descartes, God creates a union between these two different substances, a union that constitutes human nature (Cottingham and Stoothoff 1985: 22). Descartes defines substance as being that which has the capacity to exist independently from any other thing. Since Descartes envisions God as being creator of all the world, then only God exists as substance. He alone is an uncreated and independently existing thinking substance. Within the world, there are two kinds of created substances that need only God's concurrence to exist: minds, which are the limited thinking substances, and the physical world, which are finite bodies, that when taken together become infinite (Cottingham and Stoothoff 1985: 30). Apart from these two, all else that exists are merely properties of these two. The only exception to this rule is the union of the mind and body that constitutes human nature, and explains in his view, mind-body interaction (Cottingham and Stoothoff 1985: 30). .
In a more popular view, Wilhelm Leibniz's place in the history of the mind is best secured by his theory that there is no mind-body interaction. There is one a non-casual relationship of harmony between the mind and body. Leibniz is famous for his critiques, not only of materialism, but also dualism. By opposing both materialism and dualism, Leibniz created an interesting view concerning the relationship between thought and matter.