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In his "Meditations on First Philosophy", Descartes establishes a system of systematic doubt (Descartes' "Method of Doubt") which he uses to find that he can doubt everything. Everything, that is, except his own existence, as summed up by the famous expression, "cogito ergo sum" - "I think, therefore I am"1. Descartes then goes on to prove, by two methods (both cosmological and ontological) that God exists. He also demonstrates that God is incapable of causing deception.
In Descartes' sixth meditation (his last meditation), he needs to remove the spectre of the universal and systematic doubt that is still overshadowing him. Part of this is to remove the doubt he has had over the existence of corporeal things. His proof of the existence of corporeal things can be broken down into a ten-step argument:
1) There are differences between understanding, imagination, and sensory perceptions. (This implies that when I perceive something, I am not imagining it, and I can understand it).
2) Sensory perceptions are passively received (in other words, we cannot decide if we want to see or hear something if it happens to enter our eye or ear).
3) God necessarily exists and is no deceiver (this is demonstrated meditations three and five).
4) The Causal Adequacy Principle states that something cannot come from nothing, and the effect must have as much "reality"2 as the cause. This applies to ideas as well as anything else.
5) Sensory perceptions must have a cause other than me (from 2).
6) Perhaps the cause of sensory perceptions is another substance (plus 4).
7) This cause must therefore be either a corporeal substance, or God.
8) This cause cannot be God (from 3). As God is no deceiver, it would be a contradiction for Him to make me believe that I am sensing a corporeal substance when there is in fact no such substance.
9) If I sense a corporeal substance, then the cause of that perception must be the corporeal substance in question.
10) Therefore corporeal things exist.
With this argument, Descartes removes himself from his scepticism over corporeal things.
This argument is pretty neat and tidy, and follows logically. So long as Descartes' premises are true, we must accept his conclusion about the existence of the corporeal world. However, much ink has been spilt by philosophers over the past 300 or so years over whether Descartes' premises are true. The cogito argument ("cogito ergo sum") has been widely accepted, but both of Descartes' proofs of the existence of God have received great criticism. If they are false, then we cannot accept the above argument. However, as it stands, the argument works quite nicely I think. It seems to be logically valid at least.
1 - In the Meditations, Descartes actually says; "cogito sum" - "I think, I am". There is no "therefore", since this argument is not syllogistic. It is all one single inference.
2 - To Descartes, the idea of "reality" is tied in with a hierarchy of independence of existence. That which is lower down in this hierarchy depends on its existence, and has less than, or equal, reality than that which is higher up in the hierarchy. Ultimately, at the top of this hierarchy is God.
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