Substance dualism, most famously articulated by Descartes, holds that mind and body are two entirely distinct kinds of substance: res cogitans (thinking substance, unextended) and res extensa (extended, material substance). The mind is not located in space and does not obey physical laws, yet it causally interacts with the body — paradigmatically through the pineal gland in Descartes' account. This immediately generates the interaction problem: how can two substances with no properties in common causally affect each other?
Read Descartes' Meditations II and VI alongside Princess Elisabeth's correspondence with Descartes, which presses the interaction problem forcefully. Then examine occasionalism (Malebranche) and pre-established harmony (Leibniz) as historical attempts to solve the problem without abandoning dualism.
Your prerequisite, the mind-body problem, established the basic puzzle: mental states — thoughts, pains, experiences — seem different in kind from physical states like brain processes. Substance dualism is the most natural response to that puzzle and historically the most influential. Descartes arrived at it through the method of doubt you may know from Cartesian skepticism: stripping away everything uncertain, the one thing Descartes could not doubt was that he was thinking. From this he concluded that the mind — the thing that thinks — is essentially a thinking substance, and essentially *not* an extended, spatial substance. Body is just extension. The two have no properties in common, which is what makes them two substances, not two aspects of one.
The clearest way to see what substance dualism claims is by contrast with the alternatives. A monist says there is ultimately one kind of substance — either only matter (physicalism) or only mind (idealism). A property dualist says there is one substance (physical matter) but it can have two irreducible kinds of properties: physical ones and phenomenal ones. Substance dualism goes further: mind and body are not just different aspects of one thing; they are two *entirely separate* things that happen to be causally coupled. When you decide to raise your arm, your mind (res cogitans) causes your body (res extensa) to move. When you stub your toe, your body causes your mind to experience pain.
This is where the position immediately faces its most devastating objection: the interaction problem. If mind and body have no properties in common — the mind is unextended, not located in space, not subject to physical forces — how do they causally interact at all? Causal interaction, as we understand it physically, involves transfer of energy or momentum between spatially located entities. The mind, on Descartes' view, satisfies none of these conditions. Descartes' answer — the pineal gland as the site of interaction — was widely mocked even in his own time; Princess Elisabeth of Bohemia pressed him on exactly this point in their correspondence, and Descartes never gave a satisfying response. The interaction problem is the wound from which substance dualism has never recovered.
Later thinkers proposed escape routes while keeping some version of dualism. Occasionalism (Malebranche) denies direct interaction entirely: every apparent case of mind causing body (or vice versa) is actually God intervening at that moment to produce the correlated effect. Pre-established harmony (Leibniz) claims mind and body are like two synchronized clocks — they run in parallel by God's design without ever causally interacting. Both solutions preserve dualism at the cost of introducing divine intervention for every mental event. Today, substance dualism has few defenders among professional philosophers; its role is primarily as a foil against which physicalism and property dualism are developed. Understanding why it fails — specifically, why causal interaction between two substances with no common nature is incoherent — is itself illuminating about what any adequate philosophy of mind must achieve.
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