Descartes’ best argument for Dualism is best explained by a thought experiment which starts with the basic philosophical principle set forth by Gottfried Wilhelm Leibniz: If two things are identical, if they are really the same thing then anything that is true of the one must be true of the other, this is known as indiscernible of identicals. It follows that if you find two things and one has something true about it which is not true of the other then they can’t be identical.

Descartes then poses that to have absolute certainty you must dispose of everything that could possibly, in anyway, be uncertain. To throw out everything that can in anyway be subject to doubt. If you do this then what you will be left with, if anything at all, is something absolutely certain.

The evil demon thought experiment puts forth the idea that there is an evil demon tricking all of your senses by providing you with false inputs. How then can you tell if anything is certain? How do you know you are not dreaming? If the demon is manipulating all of your sensory inputs, in order to manipulate your subjective experience, then you have to disposes of all those things.

Maybe the sky isn’t blue, maybe there isn’t a sky. The movies the matrix and inception are based on this concept; how can you tell if you are dreaming? what is real? The first step is to doubt everything!

“Listen very carefully, I shall say ‘zis only once!” (Alo Alo, 1982)

Now let’s suppose you are under the influence of the evil demon, what then can we be certain of? Is there anything we can’t doubt? Can we doubt that we are doubting? No, that we can’t doubt. If you had to doubt that you were doubting, then you would in fact still be doubting. Aha! There is an absolute certainty! That as long as we are thinking then we can’t be deceived about whether we are thinking. As long as we are thinking, we exist, Cogito ergo sum – I think therefore I exist.

If I am doubting then I must have a mind, doubting and thinking is something minds do. I think, therefore I have a mind.

The next step is to tie all of this together (Pause at this point – ok – now carry on): I can doubt I have a body but I can’t doubt I have a mind. Therefore, different things are true of my body and my mind. It follows that Leibniz’s principal applies here, my mind can’t be the same as my body and therefore there must be a separation.

We would then conclude that the mind and body are two very different things, your mind can’t be your brain and what is essential to you is not your body but your mind, that the thing we want to live on forever in the afterlife is our minds, or at the very least our sense of being.