 According to this theory, the delicate fluid material called ‘animal spirits’ functions in a quasi-hydraulic machine (T 279; THM 177). Impressions on the external senses are transmitted to the ‘common sense’ in the brain, and the patterns in which animal spirits emerge from the brain in response to external stimuli are corporeal species. All voluntary or involuntary bodily actions, such as walking or blinking, are explained by the motion of animal spirits from the brain to the muscles, while internal sensations (such as hunger or thirst) are likewise experienced by the subject as a result of associated motions of animal spirits in the opposite direction, from other parts of the body to the brain. Even the passions or emotions are triggered by similar mechanisms. Thus everything that occurs in a human body may be explained by the motion of various kinds of matter in its various parts (e.g., the brain, nervous system, or the veins and arteries). The only exception in human nature to this type of physiological explanation, i.e., thought, was reserved exclusively to the mind.