It was not always obvious that brains have much to do with thinking. Aristotle believed that the
primary organ supporting thought was the heart rather than the brain, whose main function was to cool
the blood. By the sixteenth century, however, rough connections between the brain and thinking were
generally recognized—for example, that vision and hearing depend on brain anatomy. Understanding
of how brains work began only at the end of the nineteenth century when the development of new
chemical techniques for staining cells made possible identification of the cells that constitute brains.
It was decades before the electrical nature of brain cells—neurons—was appreciated. Only with the
development of computers did it become possible to formulate and test detailed hypotheses about
how the interactions among large numbers of neurons might be able to support different kinds of
thinking.