Ovidiu Babeș

Descartes' systematic physics had little to do with his quantitative accounts of natural phenomena. The former was metaphysical and was concerned with uncovering the causes operating in nature, while the latter dealt with establishing mathematical relations between various natural quantities. I reconstruct a dominant interpretation in recent literature which argues that the two practices are autonomous, and that quantitative problem-solving is normatively subordinated to metaphysical physics. However, a substantial episode of Descartes' practice resists these claims in an interesting way. Descartes' 1643 explanation of the flow of water should be, on the above reading, autonomous from metaphysically grounded physics or matter theory. Yet the explanation had unifying intentions: It is explicitly based on Descartes' laws of motion and considers the material properties of water. Additionally, because quantitative problem-solving should be subordinated to systematic physics, we would expect that Descartes' explanation is coherent with his physics of liquids. However, if we search for such a coherence, the autonomy between the two practices resurfaces as a problem. Even on a charitable reading, the physical features assumed and modelled in the 1643 explanation cannot be accounted for by Descartes' systematic physics. They are simply underdetermined in his natural philosophy. The outcome is that Descartes’ quantitative solution navigated its way around the physical constraints in a creative and opportunistic fashion.

This paper argues for a different interpretation of Roberval’s scepticism in his Aristarchi Samii de mundi systemate. Roberval’s mild sceptical attitude, along with his fake attribution of hiscosmological treatise to the ancient Aristarchus of Samos, are explained by prudential reasons related to censure. I will instead provide a more internalist reading. There are deeper metaphysical and epistemological reasons for Roberval’s pessimism about the prospect of a perfect science of celestial motions, as well as for his (non-realistic) acceptance of heliocentrism as just a more plausible system than Ptolemy’s or Tycho’s. I start by spelling out two distinct sceptical worries conflated in the Aristarchi. The first is a general agnosticism regarding certainty about the causes of the motions of the heavens—it is more of a worry that the true system of the world can never be known. The second is a particular pessimism regarding the prospects of improving astronomy. The same effect (the apparent motions of the heavenly bodies) can be produced by diverse causes. Judging by what seemed to be the most probable physical causes of the heavenly motions, Roberval saw no reason for the existence of a precisely predictable regularity in heavenly motions. Both sceptical attitudes have to do, aside from the cosmology of the Aristarchi, with the theory of science he expounds in his private Principes du debvoir et des cognoissances humaine, and in a fragment he wrote for Mersenne’s Curiouse perspective de Niceron.

This paper deals with Descartes’s and Roberval’s attempts to devise and describe the center of agitation of a composite pendulum. This episode has received some attention in the recent literature. It is usually depicted as the first step in the development of a general procedure for establishing the center of oscillation of a pendulum. My aim is to explore the different physical concepts and assumptions which informed the two mathematical accounts of the composite pendulum. I will argue that force, agitation, heaviness, or resis­tance of air essentially meant different things for Descartes and Roberval. As a result, the physical phenomena covered by the two geometrical procedures were quite distinct, and both mathematicians envisaged different roles of these phenomena within their agenda of studying nature.