Mersenne is perhaps best known as Descartes’ principal correspondent during the latter's residence in the Netherlands from 1629 until Mersenne's death. He assisted in the publication of Descartes’ Discourse on Method in 1637 and gathered the Objections to the Meditationes (1641). Mersenne also corresponded prodigiously with many other philosophers throughout Europe, acting as intelligencer and clearinghouse, communicating ideas and mediating disputes.

Mersenne was educated by the Jesuits, entering the newly founded Jesuit College of La Flèche in 1604, where he was an older contemporary of Descartes. Mersenne left La Flèche in about 1609 to study theology in Paris, at both the Sorbonne and the Collège de France. In 1611 he entered the Order of Minims, joining its Parisian convent off the Place Royale (now Place des Vosges) in 1619 and living there for the rest of his life.

Mersenne numbered Protestants as well as Catholics among his correspondents and strove to overcome their doctrinal divisions. Unlike his early, published assaults on heresy, this generally took the form of promoting a moderate, reconciliatory form of Catholicism. He never changed his perception of the dangers of skepticism, however, and his so-called mitigated skepticism amounted to a philosophical irenicism to match the religious.

Mersenne's engagement with the work of his philosophical contemporaries extended well beyond Descartes’ writings. During the 1630s, Mersenne promoted the work of Galileo; met and corresponded with Isaac Beeckman; and brought back to France from Italy in 1645 knowledge of the Torricellian barometric experiment, which led to Blaise Pascal's celebrated work on the weight of the air.

Mersenne early confronted Pyrrhonian skepticism. By the mid-1620s he had come to see it as just one example of a whole range of philosophical positions that threatened religious and social stability. The most common tactic of its opponents in France at this time was to counter Pyrrhonian arguments piecemeal, but Mersenne attacked Pyrrhonism by developing an epistemology to act as a bulwark against skepticism. Mersenne's interest in natural philosophy and in the mathematical sciences contributed to this approach.

Descartes’ view on the vacuum, or void, in nature relied on metaphysical rather than physical considerations. The Scholastic-Aristotelian denial of the possibility of a void had itself rested fundamentally on issues of principle rather than physical contingency, and despite occasional medieval arguments based on God's omnipotence, there was in practice little serious Scholastic dispute on the question in Descartes’ time. For Aristotle, place and space were interdefinitional concepts: space was always the place of something, and where nothing was in a place, there was no space there to be spoken about (see place, external versus internal). Characteristically, Aristotle happily multiplied arguments against void space despite the apparent conclusiveness of this basic metaphysical one in his Physics (bks. 4, 7). In discussing the speed of a moving body, he associated a higher speed with a greater moving force, but also related it inversely (whether strictly or not) to the resistance to motion: less resistance, faster motion; greater resistance, slower motion, all other things being equal. This enabled the observation that, counterfactually, if a body moved in a void, which by definition exerted no resistance, the body once pushed would move with infinite speed to its terminus. Since the concept of motion was necessarily sequential, as the body moves from its starting point to its end, the idea of an instantaneous translation from start to finish was incoherent. Hence, a medium with no resistance – a void – could not exist.

Descartes’ position on this subject broadly resembles the Aristotelian, in that it relies on a metaphysical argument for the void's in-principle impossibility. He holds that matter itself is nothing other than spatial extension, on the grounds that this is the only clear and distinct characteristic of matter (see clarity and distinctness). If, then, the definition of matter is identical to that of spatial extension, space itself differs in no essential respect from matter; the two are identical. Consequently, a region of space devoid of matter – a void or vacuum – is an impossibility (AT VIIIA 49, CSM I 229–30). Empirical considerations are entirely subsidiary to this argument.

The categories of “experience” and “experiment” lay at the heart of the conceptions of natural knowledge that dominated European learning at both the beginning and the end of the Scientific Revolution. The Latin words generally used to denote “experience” in both the medieval and early modern periods, experientia and experimentum, were generally interchangeable, with no systematic distinction between them except in particular contexts to be discussed; both are related to the word peritus, meaning skilled or experienced. Besides these terms and their vernacular cognates, another related Latin term, periculum (“trial” or “test”), began to be used in the late sixteenth century to designate the deliberate carrying out of an experiment (periculum facere), initially in the mathematical sciences. By the end of the seventeenth century, the construal of experience as “experiment” in this sense had acquired a wide and influential currency.

At the start of the sixteenth century, scholastic versions of Aristotelian natural philosophy dominated the approach to knowledge of nature that informed the official curricula of the universities (see the following chapters in this volume: Blair, Chapter 17; Garber, Chapter 2); Aristotle’s writings stress repeatedly the importance of sense experience in the creation of reliable knowledge of the world. Nonetheless, during the seventeenth century, many of the proponents of what came to be called by some (rather obscurely) “the new science” criticized the earlier orthodoxy of what Aristotelian natural philosophy (or “physics”) had become on the grounds that it paid insufficient attention to the lessons of experience. For example, Francis Bacon (1561–1626) wrote in his New Organon of 1620 that Aristotle “did not properly consult experience… after making his decisions arbitrarily, he parades experience around, distorted to suit his opinions, a captive.”

Historical discussion of method in the seventeenth century has long focused on the supposed development of ‘modern scientific method’, attempting thereby to explain the Scientific Revolution. However, doubt is now frequently expressed, on both philosophical and historical grounds, about the legitimacy of such an approach. Even leaving aside those arguments denying the very possibility, let alone existence, of a determinate and efficacious ‘scientific method’, the search for historical understanding through the location of its first appearance increasingly seems quixotic. As a consequence, it can now be asserted that an examination of seventeenth-century ‘method’ in the investigation of nature will have historical validity only if it respects and interprets the intellectual categories of the time — that is, if it focuses on ‘method’, not ‘methodology’. This essay, therefore, concerns ‘method’ as a logical and philosophical category; it does not purport to examine or reconstruct the procedures used by philosophers in producing new knowledge, except to the extent that these involved explicit appeal to ‘method’.

Those who talked of a ‘method’ or ‘methods’ capable of generating and organising natural knowledge usually rendered this concept plausible in the context of essentialism. Hence the world was seen as being composed of essences, or natural kinds (rather than of individuals classifiable only on conventional grounds). The essences of things, furthermore, were discoverable by rational or empirical means, and the resultant knowledge was certain rather than probable. The discoverability of such knowledge was in turn sustained by talk of effective ‘methods’. Throughout the century, therefore, debates about method formed part of broader contentions about the nature of knowledge itself.