Born in Dijon, Foucher was educated at the Sorbonne, and became a canon of the Cathedral of Dijon, although he spent his life in Paris. He was a friend of Leibniz, whose responses to Foucher’s questions provide some of the best statements of his philosophy. Foucher was a sceptic, and one of the principal 17th-century critics of both Descartes and Malebranche. His criticism of the former is retailed admiringly by Bayle in his Dictionary article on Pyrrho: ‘For if the objects of our senses appear coloured, hot, cold, odoriferous, and yet they are not so, why can they not appear extended and shaped, in rest and in motion, though they are not so?’ He was possibly the first philosopher to see that once mind is separated from the world as it is in Cartesian theory, problems of causal interaction and knowledge become insuperable, and he opened the door both to the scepticism of Bayle, and the subjective idealism of Berkeley. His principal work was the Dissertation sur la recherche de la vérité, 1673.