I knew that Descartes' claim to have thought up everything in his philosophy from scratch was false: scholars have found many elements of it in his own schoolbooks. But I had thought he was simply deceiving himself. However, based on the following, it now seems to me more likely that he was consciously promoting what he knew to be a lie:
For two months Beeckman and Descartes work closely together, and when Beeckman left Breda they kept up a correspondence... Beeckman was assured by Descartes that... "If by accident I propose something which is not contemptible, you have every right to claim it for yourself."
Years later, in 1630, Beeckman did just that. In a letter to Descartes's friend Mersenne he mentioned that some of Descartes' ideas on music had come from him. Descartes was absolutely furious and denied any influence, but when Mersenne visited Beeckman and read his journal he discovered that, indeed, many of Descartes' ideas had first been formulated by Beeckman. Descartes exploded again, telling Beeckman he had learnt as much from him as he had learnt from ants and worms... Descartes explains to Beeckman that he is mentally ill and delusional... The truth of his intellectual dependence on Beekman was utterly intolerable to him.
-- David Wootton, The Invention of Science, pp. 363-364