How he spent his time in his youth, and by what method he became so knowing, he tells the world in his treatise entitled Of Method. The Jesuits glory in that their order had the educating of him. He lived several years at Egmont (near The Hague), from whence he dated several of his books. He was too wise a man to encumber himself with a wife; but as he was a man, he had the desires and appetites of a man; he therefore kept a good conditioned handsome woman that he liked, and by whom he had some children (I think two or three). 'Tis a pity, but coming from the brain of such a father, they should be well cultivated. He was so eminently learned that all learned men made visits to him, and many of them would desire him to show them his store of instruments (in those days mathematical learning lay much in the knowledge of instruments, and as Sir Henry Savile said, in doing of tricks). He would draw out a little drawer under his table and show them a pair of compasses with one of the legs broken; and then for his ruler, he used a sheet of paper folded double. This from Alexander Cooper (brother of Samuel), limner to Christina, Queen of Sweden, who was familiarly acquainted there with Descartes.
Mr Hobbes was wont to say that had Descartes kept himself wholly to geometry, that he had been the best geometer in the world. He did very much admire him, but said that he could not pardon him for writing in the defence of transubstantiation which he knew to be absolutely against his judgment.
From John Aubrey's Brief Lives. (Edited by R Barber, Boydell Press, 1982)