Discourse, Part 5: The Rational Soul

In part five of the Discourse, Descartes discusses what makes human beings distinct from beastly animals. He begins by asserting that God originally designed the human form, including internal organs and external limbs and features, just as we know it today. He assumes, however, that God did not give souls to human bodies in the beginning.  Without rational souls, these non-thinking machines only had animal spirits, which instructed the body on how to move and follow natural, impersonal desires brought about by the senses. This is how Descartes thinks animals act. Since they neither have reason nor a significant level of intelligence, they cannot act based on personal desires or will. Everything is mechanic. What makes humans distinct from animals then is their pairing with a rational soul that allows them to speak and reason. Descartes believes that animals are incapable of communicating their thoughts, whether through words or other signs. He concludes that the inability to communicate thoughts implies the absence of thoughts. Further, animals do not understand why they perform actions. An animal will eat because of a mechanical trigger in its body that tells it to eat, while a rational man will understand his hunger and then seek food. The rational soul is what makes humans human.