Sur l'histoire du concept de vitesse d’Aristote à Galilée

Abstract

The concept of holistic measurement of motion is generally assumed to be essentially unmodified since its original formulation. I oppose this view of an ahistorical concept of holistic speed. I maintain that it originates in an anachronistic interpretation of the terminology and of the basic concepts of kinematics prior to the second half of the seventeenth century. A critical analysis of the extant texts reveals the existence of a continuous tradition in theoretical kinematics from Aristotle’s Physics up to the time of Galileo’s disciples. For our purpose the relevant feature in this tradition is its denomination of the holistic measure of motion by velocitas without further qua-lification. The two different meanings of velocitas within this tradition corres-pond to comparing motions respectively in equal times or over equal distances. The most important meaning in this pre-classical tradition is given by the expression : speeds are among themselves as the distances in equal times. This meaning plays a normative role in the tradition, and it applies both to uniform or non-uniform motions. Ignoring this tradition led to the general opinion that the statement above is inconsistent when applied to nonuniform motions, as is the case in a number of demonstrations by Galileo. Moreover, Galileo’s theorem 2 on uniform motion in the Discorsi has been used to support this view. A thorough investigation of its demonstration puts this theorem back within the framework of Galileo’s kinematic conceptions that are much akin to the pre-classical tradition.