Influenced both by his knowledge of physiology and the hydraulic (i.e., water-based) mechanics of the day, Descartes had a complex mechanical understanding of mental and physical processes that anticipated Freud’s own hydraulic model. Descartes wrote that impressions of the outside world are made on our sensory organs (i.e., eyes, ears, nose) causing animal spirits (a life giving fluid filled with purified blood) to press on our brain. The brain then sends the fluid down to our body through our nerves, causing muscles to expand and move. In this way critical functions like digestion, respiration, and even psychological processes such as sensation, the appetites and passions, take place. He also identified the pineal gland, which lies at the base of the brain, as the site where the non-physical mind and the physical body interact.