  In the Principles, Descartes defined “substance” as a thing that exists such that its existence does not depend on any other thing. He immediately added that, strictly speaking, the term applies only to God, who, as uncreated, alone depends on nothing else to exist. However, he allowed that in an extended sense it applies to things that depend only on God’s creation and continuing conservation. These created substances are really distinct from other substances insofar as they are conceivable apart from each other. They do not require a subject of inherence, and are thus ontologically, if not causally, independent. These created substances are distinguished from other things, such as qualities, which not only depend on God causally, but also depend ontologically on other things, ultimately on created substances, as subjects of inherence. In this sense, a created substance for Descartes is like the hypokeimenon of Aristotle, playing both its roles, as individuator and bearer of qualities. However, with his definition of the real distinction, he built in an unintended tendency toward monism—a tendency that Spinoza exploited. For Descartes, one thing is really distinct from another just in case it can be conceived apart from that other. But, if this test of independence is applied to causal relations, it produces the result that there is but one substance, God.