Descartes starts by doubting everything (“I will suppose then, that everything I see is spurious”) and thinks that anything which admits the slightest doubt must be false. He attempts to find something which he is unable to doubt and if he cannot he must conclude He contends that he is not able to doubt his existence. Even if there is a deceiving god who is constantly deceiving him about the world, he still must exist, as he must exist in order to be deceived. (“I am, I exist”).
He then tries to define what exactly this ‘I’ that exists is. His first answer is a ‘man’, which he defines as a ‘rational animal’. (Aristotelian answer).But this answer is far too complex for one would have to go on to define rationality and animal and these definitions would lead to other more complex ones. This is not the best way to go about answering the question.
In order to understand what ‘I’ is, he considers the things which came spontaneously into his mind when he first considered the question. His first thought is that he had a body – “That I had a face, hands, arms and the whole mechanical structure of limbs which can be seen in a corpse.” He gives the ‘I’ several characteristics – nourishment, movement, sense-perception and thinking. He attributes these to the soul and imagines this to be made of some tenuous substance like wind or ether which permeates his body. By body he means “whatever has a determinable shape and definable location and can occupy a space in such a way to exclude any other body; it can perceive by touch, sight, hearing, taste or smell and can be moved in various ways, not by itself but by whatever else comes into contact with it.” He contends that the power of self-movement, sensation and thought is foreign to the nature of the body.
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