Inspired by mechanical statues at St. Germain that moved when a person stepped on a hidden doorplate, René Descartes (1596-1650) attempted to explain all animal behavior and much of human behavior in similar terms. Descartes believed that cavities in the brain are filled with animal spirits, which he described as "a certain very fine air or wind." The cavities are connected to sense receptors in the body by hollow tubes containing delicate threads that were ordinarily taut. When a sense organ is stimulated by an external event, its thread tightens even more, opening a pore in the corresponding brain cavity, which then releases animal spirits into the hollow tube. The animal spirits flow to the muscle which then expands, causing behavior. Descartes used as an example a foot coming into close contact with a flame, which was probably the first ever description of what has come to be known as a reflex: a stimulus (heat) automatically causes a response (foot withdrawal). He claimed that the strength of an emotion is directly related to the amount of animal spirits involved in a response.