ABSTRACT
The starting-point of the classical mechanics was the conception in abstracto of a mass-point free of all external influences; not that any such thing had been or ever could have been observed, but the phenomena of movement which are observed could be most intelligibly explained in terms of a set of postulates of which the Law of Inertia was one of the most important. That the motion of such a free material point is uniform in a straight line is not subject to experimental confirmation. The nearest we could come to a practical demonstration of it would be by causing smooth balls to roll along a surface as even as we could make it, eliminating as far as was practicable the sources of friction, and observing the degree of approximation to uniform rectilinear motion which would result from the proportionate reduction of external forces. Even so, gravity and a host of other physical influences would in practice defy attempts to eliminate them altogether, and the balls themselves would not be mass-points, even though they might be assumed to be aggregates of particles. Nor could we construct on the Earth's surface a runway which is smooth and rectilinear, and if we could the result of the experiment would still not confirm our hypothesis. None of the heavenly bodies is observed to move uniformly in a straight line, and in fact if anything ever were observed to do so, it would still be questionable how far its absolute freedom from external forces could be empirically discovered. The implausibility of much current epistomology in the light of the non-empirical character of this and other important scientific principles is something we shall have to consider hereafter. At this point we need only observe that this conception of isolated, free-moving particle is the key to the entire idea of the physical world which became current during the two centuries immediately following Newton's Principia—a world of separate and independent particles brought into mutual relation by external and fortuitous causes.
