What did Descartes call the fluid in the brain's ventricles?

Descartes referred to the fluid in the brain's ventricles as "animal spirits." This concept was central to his dualistic view of the mind and body, where he believed that the mind (or soul) interacted with the physical body through the pineal gland. According to Descartes, these "animal spirits" were not just mere fluids; instead, they were thought to be a vital substance that could affect the body's movements and sensations, enabling communication between the mind and the physical world.

Descartes' idea was a significant contribution to the early understanding of how mental processes could be linked to physical states, paving the way for later developments in psychology and neuroscience. His work laid a foundation for thinking about how physiological processes correlate with thoughts and behaviors, a topic that continues to be relevant in contemporary discussions of mind-body interaction.