Force occupies a central position within classic mechanics. Seventeenth-century mechanics had restricted the use of forces to statics and the laws of impact (other motions being regarded solely as kinematic), but after Newton, mechanics linked forces and motion almost by defmition. For example, d"Alembert defined mechanics in 1765 as the science "that considers movement and motive forces, their nature, their laws and their effects within machines."² Newton's and Leibniz's work in the field of mechanics can be viewed as the motive force that moved the concept of force to center stage. However, this shift in the concept of force from the realm of everyday experience and the peripheral regions of occult phenomena to the center of mechanics did not take place without difficulty. Distinctions within the concept of force had to be introduced or refined, such as the distinctions between "inertia" and "cause of acceleration", between "momentum" and "kinetic energy" and between "contact force" and "action-at-a-distance". The process of making these distinctions was often accompanied by an extensive battle in which conflicting viewpoints were defended tooth and nail.