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The present paper examines Aristotle’s conception of cerebral functions. The first section establishes the reasons why Aristotle rejects the encephalocentric theories, popularized by some of his predecessors, which regarded the brain as the realm of psychic activities. On the basis of anatomical arguments, Aristotle concludes that the brain is devoid of any sensory or intellectual function. Yet, as the second section of this paper argues, this exclusion does not imply that the brain is of no physiological importance. In the physiology of Aristotle, the brain, the heart, and the vascularized meninx constitute an integrated system of thermic regulation based on the balance of opposites. Essential to health and responsible for the production of sleep, this system also contributes to the optimization of sensory activities, and hence to the optimization of the cognitive activities that are grounded in sensation. Examining the role of the brain in all its ramifications incidentally helps make clear how the notion of the mean, a thermic mean in this case, enables Aristotle to address the efficiency of the body as a tool of sensation.