So Descartes concludes that the only indubitable knowledge is that he is a
thinking thing. Thinking is his essence as it is the only thing about him that cannot be doubted. Descartes defines "thought" (
cogitatio) as "what happens in me such that I am immediately conscious of it, insofar as I am conscious of it".
Thinking is thus every activity of a person of which he is immediately conscious
Descartes suggested that the
pineal gland is "the seat of the soul" for several reasons. First, the soul is unitary, and unlike many areas of the brain the pineal gland appeared to be unitary (though subsequent microscopic inspection has revealed it is formed of two hemispheres). Second, Descartes observed that the pineal gland was located near the
ventricles. He believed the
cerebrospinal fluid of the ventricles acted through the nerves to control the body, and that the pineal gland influenced this process.
Finally, Descartes incorrectly believed that only humans have pineal glands, just as, in his view, only humans have minds. This led him to the belief that animals cannot feel pain, and Descartes' practice of
vivisection (the dissection of live animals) became widely used throughout Europe until the
Enlightenment. Cartesian dualism set the agenda for philosophical discussion of the
mind-body problem for many years after Descartes' death. The question of how a nonmaterial mind could influence a material body, without invoking supernatural explanations, remains controversial to this day.