This paper examines Hobbes’s criticisms of Robert Boyle’s air-pump experiments in light of Hobbes’s account in De Corpore and De Homine of the relationship of natural philosophy to geometry. I argue that Hobbes’s criticisms rely upon his understanding of what counts as “true physics.” Instead of seeing Hobbes as defending natural philosophy as “a causal enterprise … [that] as such, secured total and irrevocable assent,”1 I argue that, in his disagreement with Boyle, Hobbes relied upon his understanding of natural philosophy as a mixed mathematical science. In a mixed mathematical science one can mix facts from experience (the ‘that’) with causal principles borrowed from geometry (the ‘why’). Hobbes’s harsh criticisms of Boyle’s philosophy, especially in the Dialogus Physicus, sive De natura aeris (1661; hereafter Dialogus Physicus), should thus be understood as Hobbes advancing his view of the proper relationship of natural philosophy to geometry in terms of mixing principles from geometry with facts from experience. Understood in this light, Hobbes need not be taken to reject or diminish the importance of experiment/experience; nor should Hobbes’s criticisms in Dialogus Physicus be understood as rejecting experimenting as ignoble and not befitting a philosopher. Instead, Hobbes’s viewpoint is that experiment/experience must be understood within its proper place – it establishes the ‘that’ for a mixed mathematical science explanation.
HobbesMan and Citizen42; ol ii.93. My focus in the present paper is upon the relationship between the natural philosophy of Dialogus Physicus and Part iv of De Corpore and geometry; however Hobbes also uses principles from first philosophy in natural philosophy. Douglas Jesseph (“Hobbesian Mechanics” Oxford Studies in Early Modern Philosophyiii : 119–152 here 138–139) sees two parts to natural philosophy: first a priori principles of first philosophy (what Jesseph calls the “persistence principle” in De Corpore 8.19 and the principle of “action by contact” in De Corpore 9.17); and second the use of these principles within natural philosophy by hypothesis (e.g. ol i.339; ol i.354; ol i.417). In addition to these two principles Hobbes also borrows other principles from first philosophy such as a principle related to the division of bodies and places used in De Corpore 25.6 (ol i.321) and another related to the necessity of an effect following from a necessary cause used in De Corpore 25.13 when explaining deliberation (ol i.333). A complete account of Hobbes’s claim that “something also being demonstrated a priori” will include both first philosophy and geometry.