The philosophy of mind deals with issues related to questions about the mind. These issues may be metaphysical, or they may be epistemological. Davies states that the metaphysical questions about the mind include the nature of the mind as a substance and how mental phenomena have a place in the causal order. The most fundamental of these questions is the mind-body problem. Epistemological questions include how we can come to any knowledge of minds other than our own (Davies 251).
One answer to the mind-body problem is dualism. In the Meditations on First Philosophy, René Descartes discusses what has come to be called the Cartesian dualism, which refers to the theory offered by Descartes that the mind and body are separate and that the mind is incorporeal:
Throughout his life Descartes firmly believed that the mind, or soul. . . was essentially nonphysical. . . The thesis of the incorporeality of the mind seems, from first to last, a fixed point in Descartes' thinking (Cottingham 236).
The one thing that cannot be doubted and that is true each time it is expressed by a person is that that person exists. Descartes finds that he might doubt everything else because his senses may deceive him. He can thus deny that he has a body and senses because he perceives these things only through what he has called the senses, and all this data might be false. He asks then if it is possible that he can exist without the body and without the senses, and of course he can because the one thing he knows without the senses is that he exists. He exists in his mind, and he knows that he exists through the awareness of that mind. Even if there is some powerful force bent on deceiving the observer, the observer knows that he himself exists:
Let him deceive me as much as he will, he can never make me be nothing as long as I think that I am something. Thus, after having thought well on this matter, and after examining all things with ...