The chapter presents a contradiction on the disposition of Descartes as a scholar. First, the chapter states that Descartes believes in knowledge as the clear and distinct perception of propositions by the intellect; knowledge in the strictest sense is certain, indeed indubitable, and grounded in the purely rational apprehension of truth. But it is also generally recognized that Descartes was a serious experimenter, at least in his biology and his optics, and that in these areas, at least, he seemed to (...) hold that knowledge requires an appeal to experience and experiment. To the 20th-century philosopher this looks a bit puzzling: how can Descartes be both a rationalist, who sees knowledge as deriving from the intellect, and an experimentalist, who sees experiment and observation as essential to the enterprise of knowledge? This is the puzzle that the chapter would like to address. It argues that not only is there no contradiction here, but that the appeal to experience is an essential part of the method for constructing a deductive science. (shrink)