The Principle of Inertia

It is sometimes asserted that this principle was known to antiquity. ⁽¹⁾ It is certain that in following the exposition of a Greek atomic system, such as that of Democritus, through the refutations of Aristotle, or that of Epicurus in De Rerum Natura:, a modern reader is almost without fail led to believe that these philosophers implicitly postulated inertia. We do not, however, believe that such was the case. Nowhere, either in an atomistic philosopher or even in any ancient writer, does there appear any allusion to a belief in an indefinite motion in a straight line by virtue of an impetus received and without the continued action of a force. It is probable that an ancient atomist, if asked the reason for the continuous motion of particles, would have replied that they fall or move in virtue of a force inherent in them. This is plainly seen in Lucretius ; and Democritus seems to have been of the same opinion. ⁽²⁾ At the most he would have referred to the example of some known persistent motion, such as that of the top or of the vibrating cord, ⁽³⁾ rather than the motion of a projectile which would appear to him doubtless as a motion toward an object and thus necessarily limited. Had it been otherwise, Aristotle, in expounding his theory of the natural circular motion of celestial bodies, would have been constrained to allude to it. We well know that he takes great pains to refute the opinions of the atomists. Now, his exposition seems to prove, on the contrary, that he had no adverse opinion to combat on that point. ⁽⁴⁾ It might be objected that this argument is not absolutely convincing as regards terrestrial bodies: for Aristotle, indeed, the two kinds of motion are very different, and, strictly speaking, they might have been so also (although this is little likely) for the atomists. But another passage of the Physics seems decisive in this respect. Aristotle uses the impossibility of continuous motion in a straight line as a proof by reductio ad absurdum; this impossibility seems so evident to him that he does not even suppose that there can be a difference of opinion on this point. ⁽⁵⁾ He certainly would have expressed himself quite differently if the principle of inertia had been asserted even implicitly, especially if it had been formulated by Democritus.