Dans les Principia philosophiae, publies pour la premiere fois en latin en 1644, Descartes expose sous une forme synthetique ce que les analyses des Meditations et du Discours avaient mis a jour, a savoir l'articulation entre les principes generaux de la connaissance humaine, les principes des sciences et ses decouvertes sur la structure du monde physique. C'est dans la Lettre-preface a son traducteur francais, l'abbe Picot, qu'apparait aussi la celebre conception de la philosophie comme une arborescence, a partir des racines (...) metaphysiques, en un tronc de la physique puis trois branches principales que constituent la medecine, la mecanique et la morale. Filant la metaphore, Descartes vise le dernier degre de la sagesse comme le fruit ultime de cette croissance organique du savoir: Or comme ce n'est pas des racines, ni du tronc des arbres, qu'on cueille les fruits, mais seulement des extremites de leurs branches, ainsi la principale utilite de la philosophie depend de celles de ses parties qu'on ne peut apprendre que les dernieres. (shrink)

Most modern analysts of Newton's laws of motion, whether they have approached the subject from a historical or from a philosophical viewpoint, have tended to concentrate on the status of the first two laws; the third law has largely been overlooked, or else it has been dismissed as somehow less interesting. My purpose in this paper is to reverse this approach—I intend to investigate some of the historical aspects of the third law, particularly the empirical background to Newton's statement of (...) it, and in so doing, I intend to skirt most of the questions which have been raised concerning the status of the other two laws. In concentrating on the historical aspects of the third law, I shall also by-pass Mach's controversial re-interpretation of its role in mechanics, for while Mach saw the law as the basis for an operational definition of “mass”, it is quite clear that Newton did not so regard it. On the contrary, Newton seems to have regarded all three of his laws as straightforward statements of fact about the world, so that a knowledge of the factual background to the laws is a fundamental pre-requisite to an understanding of Newton's thought. (shrink)