 Rousseau's ideas were the result of an almost obsessive dialogue with writers of the past, filtered in many cases through conversations with Diderot.In 1749, Rousseau was paying daily visits to Diderot, who had been thrown into the fortress of Vincennes under a "lettre de cachet" for opinions in his ""Lettre sur les aveugles"", that hinted at materialism, a belief in atoms, and natural selection.According to science historian Conway Zirkle, Rousseau saw the concept of natural selection "as an agent for improving the human species."