straight line, unless it is compelled to change that state by force impressed upon it." This is called the principle of inertia (lex inertiae).

The second edition of this work was published in 1713, while Newton was still alive. It included an extended preface by Cotes, then professor at Cambridge. In it Cotes says about this basic principle: "It is a law of nature universally received by all philosophers."

Students of physics do not puzzle over this law today and have not for a long time. If we mention it at all and know anything about it, that and to what extent it is a fundamental principle, we consider it self-evident. And yet, one hundred years before Newton at the apex of his physics put this law in this form, it was still unknown. It was not even Newton himself who discovered it, but Galileo; the latter, however, applied it only in his last works and did not even express it as such. Only the Genoese Professor Baliani articulated this discovered law in general terms. Descartes then took it into his Principia Philosophiae and tried to ground it metaphysically. With Leibniz it plays the role of a metaphysical law (C.I. Gerhardt, Die philosophischen Schriften von G. W. Leibniz [Berlin, 1875-1890], IV, 518, contra Bayle).

This law, however, was not at all self-evident even in the seventeenth century. During the preceding fifteen hundred years it was not only unknown, but nature and beings in general were experienced in such a way that it would have been senseless. In its discovery and its establishment as the fundamental law lies a revolution that belongs to the greatest in human thought, and which first provides the ground for the turning from the Ptolemaic to the Copernican conception of the universe. To be sure, the law of inertia and its definition already had their predecessors in ancient times. Certain fundamental principles of Democritus (460-370 B.C.) tend in this direction. It has also been shown that Galileo and his

* Isaac Newton, Mathematical Principles of Natural Philosophy and His System of the World, Andrew Motte, trans., 1729; revised translation, Florian Cajori (Berkeley: University of California Press, 1946), p. 13—TR.