The two operations of our understanding, intuition and deduction, on which alone we must rely in the acquisition of knowledge. ~ René Descartes
French philosopher and mathematician René Descartes (1596–1650) advocated the fluid-mechanical theory of brain functioning: that the brain acted as a pump, albeit with a Christian twist.
The brain-pump theory could explain animal behavior, but Descartes considered it inconceivable that cranial fluid mechanics could account for the full range of human behavior. Descartes reckoned that people possess intellect and the ability to reason unlike other animals. Beyond that, only humans had a God-given soul. The unique mental and spiritual gifts that made humans above all other animals resided outside the brain, in the mind. Descartes surmised that brain-based functions in humans applied only to those behaviors that resembled that of the beasts.
Touting dualism, Descartes believed the mind was a separate spiritual entity, receiving sensations and commanding brain-directed movement via the pineal gland, which was the “seat of the soul.” The pineal gland is now known to regulate biorhythmic functions, such as the circadian rhythm (wake/sleep cycle), by production of the hormone melatonin. Production of melatonin in the pineal gland is inhibited by being in sunlight and stimulated by darkness.