Marin Mersenne was central to the new mathematical approach to nature in Paris in the 1630s and 1640s. Intellectually, he was one of the most enthusiastic practitioners of that program, and published a number of inºuential books in those important decades. But Mersenne started his career in a rather different way. In the early 1620s, Mersenne was known in Paris primarily as a writer on religious topics, and a staunch defender of Aristotle against attacks by those who would replace him by a new philosophy. In this essay, I would like to examine Mersenne's changing attitude toward Galileo. In the early 1620s, Mersenne lists Galileo among the innovators in natural philosophy whose views should be rejected. However, by the early 1630s, less than a decade later, Mersenne has become one of Galileo's most ardent supporters. How, then, did Mersenne learn to love Galileo?

Important to Kuhn's account of scientific change is the observation that when paradigms are in competition with one another, there is a curious breakdown of rational argument and communication between adherents of competing programs. He attributed this to the fact that competing paradigms are incommensurable. The incommensurability thesis centrally involves the claim that there is a deep conceptual gap between competing paradigms in science. In this paper I argue that in one important case of competing paradigms, the Aristotelian explanation of the properties of bodies in terms of matter and form as opposed to the Cartesian mechanist paradigm, where the properties of bodies are explained on the model of machines, there was no such conceptual gap: the notion of a machine was as fully intelligible on the Aristotelian paradigm as it was on the Cartesian. But this does not mean that the debate between the two sides was conducted on purely rational terms. Rational argument breaks down not because of Kuhnian incommensurability, I argue, but because of other cultural factors separating the two camps.

Recent literature has explored at some length the transition between individual observations and the experimental facts that they are supposed to establish, emphasizing particularly the social dimension of this question. In this article I examine some crucial stages in the history of this problem, in particular, the way in which the establishment of experimental facts became social. I begin with a brief discussion of experimental facthood in late Renaissance thought before turning to Bacon and Descartes and showing the extent to which their conception of experimental facthood is radically individualistic. I then discuss the self-consciously social conception of experimental facthood found in the writings of the early Royal Society. After a digression about some recent issues concerning the rhetoric of scientific experiments in the period, I end with some speculations about why the transition occurred when it did. The transformation in the philosophical view about the role of community in the establishment of experimental facts, I suggest, is closely connected with the emergence of a community entitled to make the judgments necessary to establish such facts .