History of Western Philosophy - Bertrand Russell [321]
Moreover there are sometimes two different ideas of the same external object, e.g. the sun as it appears to the senses and the sun in which the astronomers believe. These cannot both be like the sun, and reason shows that the one which comes directly from experience must be the less like it of the two.
But these considerations have not disposed of the sceptical arguments which threw doubt on the existence of the external world. This can only be done by first proving the existence of God.
Descartes's proofs of the existence of God are not very original; in the main they come from scholastic philosophy. They were better stated by Leibniz, and I will omit consideration of them until we come to him.
When God's existence has been proved, the rest proceeds easily. Since God is good, He will not act like the deceitful demon whom Descartes has imagined as a ground for doubt. Now God has given me such a strong inclination to believe in bodies that He would be deceitful if there were none; therefore bodies exist. He must, moreover, have given me the faculty of correcting errors. I use this faculty when I employ the principle that what is clear and distinct is true. This enables me to know mathematics, and physics also, if I remember that I must know the truth about bodies by the mind alone, not by mind and body jointly.
The constructive part of Descartes's theory of knowledge is much less interesting than the earlier destructive part. It uses all sorts of scholastic maxims, such as that an effect can never have more perfection than its cause, which have somehow escaped the initial critical scrutiny. No reason is given for accepting these maxims, although they are certainly less self-evident than one's own existence, which is proved with a flourish of trumpets. Plato, St Augustine, and St Thomas contain most of what is affirmative in the Meditations.
The method of critical doubt, though Descartes himself applied it only half-heartedly, was of great philosophic importance. It is clear, as a matter of logic, that it can only yield positive results if scepticism is to stop somewhere. If there is to be both logical and empirical knowledge, there must be two kinds of stopping points: indubitable facts, and indubitable principles of inference. Descartes's indubitable facts are his own thoughts—using 'thought' in the widest possible sense. 'I think' is his ultimate premiss. Here the word 'I' is really illegitimate; he ought to state his ultimate premiss in the form 'there are thoughts'. The word 'I' is grammatically convenient, but does not describe a datum. When he goes on to say 'I am a thing which thinks', he is already using uncritically the apparatus of categories handed down by scholasticism. He nowhere proves that thoughts need a thinker, nor is there reason to believe this except in a grammatical sense. The decision, however, to regard thoughts rather than external objects as the prime empirical certainties was very important, and had a profound effect on all subsequent philosophy.
In two other respects the philosophy of Descartes was important. First: it brought to completion, or very nearly to completion, the dualism of mind and matter which began with Plato and was developed, largely for religious reasons, by Christian philosophy. Ignoring the curious transactions in the pineal gland, which were dropped by the followers of Descartes, the Cartesian system presents two