11 - New doctrine of motion and rest and the conclusions associated with it in the fundamental principles of natural science while at the same time his lectures for this half-year are announced (1758)

Summary

EDITOR'S INTRODUCTION

On 1 April 1758 Kant published a short essay on motion and rest with the continued hope of attracting more students to his lectures by giving a clear illustration of how he approaches the fundamental principles of mechanics much as he had presented objections to a commonsense account of certain meterological phenomena almost a year earlier. In this piece, Kant presents an attack first on the concept of absolute motion and then on a conception of inertia that rests on absolute motion. The attack on absolute motion – Newton is nowhere mentioned by name and Newton's arguments for absolute motion and absolute space are also not discussed, so it is uncertain whether Newton was his intended target – proceeds from the fact that when we judge whether an object is at motion or at rest it is always with respect to other objects; we cannot perceive absolute motion by perceiving absolute space, nor can we treat the fixed stars as an absolute reference frame, since they could be moving with respect to even more distant objects. As a result, the notion of motion we employ is not absolute, but relative. Moreover, the notion of relative motion that we use in such contexts is one according to which relative motion is reciprocal and equal. That is, if A moves three units closer to B in the time interval of t₀ to t₁, then B must also move three units closer to A during that interval. Based on this analysis of motion, Kant infers two corollaries, namely that no body can collide with another body that is at absolute rest (since it must be moving towards the first body), and that action and reaction are equal in the collision of bodies.