  Having separated the mind so sharply from the body, Descartes is left with having to explain their evident unity. More specifically, he is burdened with trying to explain how two really distinct substances seem to interact causally. Their causal interaction seems problematic because, according to Descartes, each substance is independent; the infinite substance depends on nothing but itself (Principles, I, §51, CSM, I, p. 210, AT 24), while created substances depend on nothing but God for their existence (Principles, I, §52, CSM I, p. 210, AT 25). If distinct substances interact causally then they seem to depend on one another, and this would go against their nature qua substances. This is why the union of the mind and body is a thorny issue for Descartes, and was and continues to be a source of much debate (Cf. for example, Hoffman, 1986). For some, a version of this problem translates into Spinoza’s metaphysics (cf. Section  1.9.4). The issue of the nature of the “real distinction” for Spinoza is discussed in the subsequent section.