Decades before Newton’s Principia ushered in the age of modern science, Aristotelian physics faced a serious challenge against its weakest point, in the quest to construct a new theory of projectile motion. Yet how were such new principles of motion conceived, without reference to an established theory of mechanics? This paper explores the conceptual space between the rejection of Aristotle’s physics and the appearance of Newton’s physics in which people such as Harriot and Galileo sought new ways to understand mechanical phenomena. This paper seeks to show that both Harriot and Galileo looked to the (early seventeenth-century) zeitgeist of accepting the law of reflection as a law of oblique projection-rebound for all material bodies, for their foundational (non-Aristotelian) conceptual resource. The kinematics and dynamics of light’s compound, oblique motion had been developed over centuries within optics, and constituted the ‘ideal’ resource for novel analyses of oblique projectile motion. Whereas Harriot’s inclusion of air resistance restricted the generalizability of optical principles to his simplest case, the upright parabola, Galileo’s idealized analysis of projectile motion in a vacuum represented all cases of projectile motion as upright parabolas. He was thus able to generalize these optical principles to projectile motion per se.
This paper aims to show that the seventeenth-century conception of mechanics as the science of particles in motion founded on universal laws of motion owes much to the employment of a new conceptual resource – the physics of motion developed within optics. The optical analysis of reflection was dynamically interpreted through the mechanical analogy of rebound. The kinematical and dynamical principles so employed became directly applicable to natural phenomena after the eventual transformation of light's ontological status from that of an Aristotelian ‘quality’ to a corpuscular phenomenon, engendered by the rise of atomism during the first half of the seventeenth century. The mechanization of light led to a conceptual shift from the analogical employment of dynamical principles in the physical interpretation of reflection to the mechanical generalization of optical principles – the direct application of kinematical and dynamical principles of reflection to mechanical collisions. This first part of the paper traces out the first conceptual shift from Aristotle's original analogy of reflection as rebound to its full concretization. A second part will trace out the second conceptual shift, from the full concretization of this analogy to the axiomatization of already generalized kinematical and dynamical principles of reflection into laws of nature and of motion.
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