As Descartes fully understands, the new matter theory explains the behavior of sensible bodies by reference to imperceptible particles. So the question arises, how can we arrive at the knowledge of the shapes, sizes, and motions of these particles? The answer involves the epistemic status of hypotheses, but the role of hypotheses in Descartes’s philosophy is not clear, or it seems to have undergone some change, and the Cartesians do not seem to have accepted Descartes’s view fully. Now, it has already been pointed out that Descartes was not as a prioristic about scientifi c method and the use of hypotheses as is usually thought, or at least that he became less so in his later years (see, for example, Blake, Ducasse, and Madden 1960; Laudan 1981; Garber 1978, and 2001b; and Clarke 1982), and that the Cartesians, while maintaining Descartes’s propensity for mechanistic explanations, became more empirical and pursued aggressively a quasi-hypothetical-deductive method (Clarke 1989; McClaughlin 2000; also Ariew 2006). But the motivations for these shifts are not clear: it is not useful to treat Descartes and the Cartesians as sleepwalkers, darkly perceiving the hypothetico-deductive nature of science, as has sometimes been done. Science may or may not have a single method; hypotheticodeductivism may or may not be that method. Even if it were, this in itself could not explain why the Cartesians accepted some form of it, if they did. So I wish to investigate the various uses Descartes and the Cartesians made of hypotheses and the reasons they gave for those uses. In this chapter I limit my discussion to Descartes and some of his immediate predecessors. As will become obvious, a fair portion of my discussion concerns Descartes’s notion of moral certainty, which Descartes uses to distinguish between nonhypothetical fi rst principles about general things and hypothetical ones about particular things. Descartes’s usual view was that his hypotheses could be grounded in nonhypothetical, self-evident principles, that he had or could provide such a derivation. By Part IV of the Principles of Philosophy, he knew that such a demonstration would be futile. Descartes’s opinion in the Principles is that his hypothetical principles are not absolutely but merely morally certain, meaning that

there is at least some logical connection and coherence in them, such that his physics would have to be rejected and taken only as a fi ction, or else it all has to be accepted and not be rejected until another is found more capable of explaining all the phenomena of nature. The key concept for Descartes is thus “moral certainty,” a term he consciously borrows from the late scholastics. I try to make sense of Descartes’s concept by reference to contemporary scholastics texts, namely those of Roderigo Arriaga, Eustachius a Sancto Paulo, and Francisco Suárez. However, because of how Descartes distinguishes between absolute and moral certainty, to draw the full picture, I would have to discuss as well the status of the method of doubt in the second half of the seventeenth century. In brief, my overall argument would have been that the rejection of the method of doubt, which is the underpinning of moral certainty, causes many Cartesians no longer to distinguish between the absolutely and the merely morally certain-between that which we cannot doubt and that about which we have no doubt although we could doubt it-and thus to treat all principles on a par with one another. But I leave this extension of my inquiry to another occasion.