Once upon a time, Rene Descartes sat down in his armchair by the fire, closed his eyes and began to discard every belief he had that he could doubt, hoping to retain only those beliefs that could not be doubted – and that he could therefore hold with certainty.
Since we can easily doubt the senses – it is possible that we are just brains in vats, without a body, being manipulated by an evil demon – Descartes had to discard everything he had ever learned by experience. But if our senses give us only illusions, what can we know at all? Descartes concluded that the only thing we cannot doubt is our own existence. Because if we are indeed manipulated and deceived, then there must be someone that is deceived; there must exist a thinking subject. Hence, Descartes concluded: cogito ergo sum – I think, therefore I am.
Since it is possible to doubt the existence of the world, but not the existence of the thinking subject, Descartes concluded that thinking must be the essence of the mind: the mind is a thinking substance.
Initially, Descartes arrived at his mind/body dualism via a fallacy. Because he couldn’t doubt the existence of the mind as a thinking substance, but he could doubt the existence of the body (or any material thing), these things must be distinct, he thought. But he realised that this doesn’t follow. Just because he has knowledge of the mind, but not of the body, doesn’t mean they are distinct. Therefore, Descartes offered another argument: since the essence of material substances is that they are extended, and extended things cannot think, the mind must be unextended, and therefore immaterial. The body is extended, therefore material, therefore the body must be distinct from the mind. Voila: not only mind/body dualism, but also substance dualism (the view that there are two kinds of substances: material and immaterial).
But one woman wasn’t buying any of this. Descartes corresponded with Elisabeth, Princess of Bohemia, over an extended period of time. Elisabeth was well-read in philosophy, and not easily swayed by Descartes. In one of her letters, she asked an important question: how is it possible that the mind interacts with the body (and also vice versa: that the body interacts with the mind), if they are distinct substances?
Descartes admits that his answer to this question is not satisfactory, and his followers have struggled with it ever since. The aim of this lesson is to appreciate the nature and force of this interaction problem, as it is a problem one has to face if one, for instance, wants to accept an idea of a free will, or an idea of an immortal soul.