  After publication of the Discourse in 1637, Descartes received in his correspondence queries and challenges to various of the doctrines, including his account of the sequence of phenomena during heart-beat and the circulation of the blood; his avoidance of substantial forms and real qualities; his argument for a distinction between mind and body; and his view that natural philosophical hypotheses could be “proven”  through the effects that they explain (6:76). Descartes' correspondence from the  second half of the 1630s repays close study, among other things for  his discussions of hypothesis-confirmation in science, his replies to objections concerning his metaphysics, and his explanation that he had left the most radical skeptical arguments out of this work, since it was written in French for a wide audience (1:350, 561).