Reason distinguishes humans from animals and machines for Descartes, but not for Hume. Descartes treats rational thought as uniquely human, whereas Hume argues that animals also infer from experience, so the difference is mainly one of degree.
Descartes: Reason as Uniquely Human
Descartes' position rests on substance dualism. Humans possess an immaterial thinking mind, or res cogitans; animals are material mechanisms governed by physical processes.
In the Discourse on Method, Descartes offers two tests for distinguishing a rational human from a machine. First, machines cannot use language creatively to express thoughts across unlimited circumstances. Second, although a machine may excel at tasks, it cannot respond appropriately generally through universal reason. Animals also lack genuine thought; their intelligent behaviour is mechanical rather than rational. Humans are therefore distinguished by a rational soul.
Hume: Reason Is Not Exclusively Human
Hume rejects reason's exclusivity. In the Enquiry Concerning Human Understanding, he argues that humans and animals learn by observing repeated patterns and expecting similar outcomes. A dog anticipates food or avoids pain because experience creates habits of expectation.
| Thinker | View of animals | Human-animal difference |
|---|---|---|
| Descartes | Non-rational automata | Difference in kind: humans possess immaterial minds, language, and universal reason |
| Hume | Capable of experiential inference | Difference in degree: humans show greater complexity and reflection |
For Hume, such inference arises from custom or habit, not demonstrative reason. Human cognition is thus continuous with animal cognition. Although Hume does not present Descartes' machine tests, his naturalistic account undermines a sharp metaphysical boundary based on reason.
