Abstract: Although Galileo's physical science of impact was not completed and published in his lifetime, his theories were very influential throughout Europe. Particularly influential was his claim that the force of impact (which he called percussive force) is demonstrably infinite. Appealing to a number of empirical experiments, Galileo and his followers compared the effects of percussive focus (exerted by bodies in motion) and the effects of gravity (exerted by stationary bodies). They argued that because moving bodies can always produce a greater effect than stationary ones, the percussive force must infinitely exceed the force of gravity. Though often forgotten in accounts of the history of mechanics, this doctrine was both widely accepted (with notable endorsements from Torricelli and Leibniz) and stands as one of the immediate precursors to Newton's mechanics. The doctrine is also of great philosophical interest, since lurking beneath the surface are various assumptions about the nature of force and its measurement, and about the relationship between metaphysics, mathematics, and natural science.

George Berkeley was one of the first known critics of this doctrine, and he leveraged a number of arguments against the Galileans' purported demonstrations. The aim of this paper is to evaluate the strength of Berkeley's critique. Although the critique is not itself immune to criticism, I argue that in one important part, Berkeley's criticism was exactly right, and right in a very interesting way. I argue that Berkeley correctly identifies that the Galieleans' theory of force, which holds that a body's gravity is intrinsic to the body, was responsible for their absurd conclusion. Given that a body's gravity is intrinsic to it, the Galileans supposed that one could compare the effects of stationary bodies with the effects of bodies in motion, because both continue to exert some force. Given this comparability and the effects of percussion observed in experiment, the Galileans concluded that the force of percussion is infinite. Berkeley's contribution was to argue against the comparability of the forces (because this was based on a faulty metaphysical doctrine of force), and therefore against the infinite force of percussion. If I am right, this illustrates one important way that Berkeley's instrumentalism in dynamics and idealism in metaphysics played an important role in the history of dynamics, even if his arguments have to this point not received the attention they require.