Mind

To many readers, the relationship between the brain and mind may be obvious, but that has not always been the case. In 1652, for example, the philosopher Henry More (1614–1687) declared that the brain "is no more capable of thought than a cake of suet or a bowl of curds" (Zimmer, p. 5).

Medieval and Renaissance physicians sought to understand the mind with a mix of Christian theology and Greek philosophy. They believed the body, for example, was divided into three anatomical regions, each designed for its own soul. The vegetative soul in the liver was responsible for desires and appetites. The heart housed the vital soul, which produced passions and action.

The rational soul, not surprisingly, was a more complicated matter. Since it was immaterial and immortal, it could not reside in one specific place in the body. But its faculties—such as reason, memory, and imagination—were believed to be carried out by "animal spirits" that supposedly swirled within three hollow chambers in the head known as the ventricles.

Anatomy, then, was the study of the houses of the souls. But anatomy alone was not enough to account for the life of the mind. Physicians also had to understand the fluids that coursed through the body. Known as the four humors (black bile, yellow bile, blood, and phlegm), these humors needed to be balanced for good health; if they fell out of equilibrium, they brought disease. Humors also gave each individual his or her temperament, be it the sad detachment of melancholy or the swift rage of choler. As the humors became corrupted or moved to the wrong place in the body, they could cause epilepsy or alter the temperament, even leading to madness. Physicians sought to cure many psychological disorders by bringing the humors back in balance, typically with bleeding and purging, or by applying herbs.

During the Renaissance, these theories of souls and humors were vigorously debated. And yet in all these arguments, the brain was strangely absent. The substance of the brain—now recognized as consisting of billions of neurons trading complex signals—was seen as nothing more than phlegm. This is understandable when one considers how delicate the brain is. Without preservatives or refrigeration, a brain quickly decays after death, while muscles and bones remain available for further study.