Descartes indivisibility argument for substance dualism Michael Lacewing

Descartes’ indivisibility argument for substance dualism Michael Lacewing enquiries@alevelphilosophy. co. uk (c) Michael Lacewing

Substance dualism • Substance dualism: there are two sorts of substance, mind (or soul) and matter – Minds are distinct from bodies – Minds are not properties of bodies – Mental properties are properties of a mental substance • Cartesian substance dualism: minds can exist independent of bodies (c) Michael Lacewing

Descartes’ indivisibility argument • The body is extended in space; it has (literal) parts. • The mind has no (literal) parts. • Leibniz’s law of the indiscernibility of identicals: If X and Y are the same thing, then they have the same properties – Therefore, if X and Y have different properties, they are not the same thing • Therefore, mind and body are different.

The mental is divisible • Mental illness and theories of the unconsciousness suggest that the mind does have ‘parts’ • Reply: bodies are spatially divisible, but minds are only functionally divisible

Not everything physical is divisible • Is it always true to say that something physical has parts? – Could the smallest physical things not be divided in principle, e. g. force fields? – But we can still talk of them having half the size – This depends on the best theory of space • If not all physical things are divisible, then the fact that something, e. g. the mind, isn’t divisible doesn’t show that it isn’t physical

Are minds substances? • Suppose minds are not substances – Suppose there are only mental properties – Minds are neither divisible nor indivisible – Properties aren’t divisible – only substances literally have parts • Descartes has assumed that the mind is a substance to show that it is an indivisible substance