Rene Descartes on the mind/body problem, or philosophical dualism. In more ways than one, Descartes' distinction between the human mind and the human body is the starting point for modern philosophy with its insistence on the important of epistemological problems -- problems that in Kant's day focused on the transcendental possibility of knowing, and in more contemporary philosophies tend to focus on the physical possibility of mentality (i.e., philosophy of mind).
Descartes conceived of his own thinking as distinct from the physical activity of his body -- the I, according to Descartes, is a thinking thing, a thing that thinks. The thinking I is connected to the body only through the pineal gland a small, suspended, organ in the middle of the brain that 'can move in as many ways as the body can move', and represents for the body, the thought of the mind. On Descartes' view, mind and body are separate substances. His metaphysics is, therefore, dualistic, insofar as substances are (according to the thought of the day) self-complete and self-caused things.
"It is certain that I, that is to say my mind, by which I am what I am, is entirely and truly distinct from my body, and may exist without it" (Meditations, 156).
"We perceive bodies only by the understanding which is in us, and not by the imagination, or the senses... we do not perceive them through seeing them or touching them, but only because we conceive them in thought" (Meditations, 112).