Matter is divisible into parts while the mind is indivisible.
Material beings perish while the soul is immortal.
Matter is extended while the mind is a thinking thing. The mind and the brain are different kinds of substances. The mind effects the whole body and not just the brain. Descartes causal principle says that there must be some intelligible connection between the cause and the effect, but in this case that is not possible. Descartes holds that the minds cannot directly effect the animal spirits.