Marin Mersenne represents a new seventeenth-century perspective on natural knowledge. This perspective elevated the classical mathematical sciences over natural philosophy as the appropriate models of what can be known, of how it can be known and of the cognitive status of that knowledge. His early publications had the apologetic aim not only of combating various forms of heresy, but also of opposing philosophical scepticism, which was widely regarded in Catholic France of the early seventeenth century as undermining the certainty of religious dogma. To that end, Mersenne stressed the certainty of demonstrations in sciences such as optics, astronomy and mechanics, all of which stood as ‘mathematical’ sciences in the classifications of the sciences stemming from Aristotle. Mersenne’s stress on the mathematical sciences contrasted them with natural philosophy in so far as the former concerned only the measurable external properties of things whereas the latter purported to discuss their inner natures, or essences. In accepting the considerable degree of uncertainty attending knowledge of essences, and juxtaposing it to the relative certainty of knowledge of appearances, Mersenne adopted a position (since called ‘mitigated scepticism’) that combated scepticism by lowering the stakes: in accepting that the essences of things cannot be known, he agreed with the sceptics; but in asserting that knowledge of appearances can, by contrast, be had with certainty, he rejected the apparent intellectual paralysis advocated by the sceptics. In furthering this programme, Mersenne embarked on a publication effort relating to the mathematical sciences, combined with a massive lifelong correspondence on largely philosophical as well as religious topics with a wide network of people throughout Europe.
