  The designation “laws of motion” had been used in the Philosophical Transactions of the Royal Society in the late 1660s for principles governing motion under impact put forward by Christopher Wren, John Wallis, and Christiaan Huygens. Only the first of the three laws Newton gives in the Principia corresponds to any of these principles, and even the statement of it is distinctly different: Every body perseveres in its state of being at rest or of moving uniformly straight forward except insofar as it is compelled to change its state by forces impressed. This general principle, which following the lead of Newton came to be called the principle or law of inertia, had been in print since Pierre Gassendi's De motu impresso a motore translato of 1641. Newton probably first encountered it in print when he read Descartes' Principia, where it is comprised by his first two “laws of nature” and is used immediately to assert “that any body which is moving in a circle constantly tends to move away from the center of the circle which it is describing.” This is the basis for Descartes concluding that some form of unseen matter (namely the vortices) must be in contact with the planets, for otherwise they would go off in a straight line. It is the first of the three hypotheses from which Huygens develops his theory of “falling heavy bodies and their motion in a cycloid” in his Horologium Oscillatorium of 1673: If there were no gravity, and if the air did not impede the motion of bodies, then any body will continue its given motion with uniform velocity in a straight line. Newton had adopted it as a “hypothesis” in the registered version of “De Motu,” though stated without reference to impressed forces: Every body under the sole action of its innate force moves uniformly in a straight line indefinitely unless something extraneous hinders it. The striking difference in the formulation in the Principia versus the one in “De Motu” — and, for that matter, versus all earlier formulations in print — is the reference to impressed forces. In all earlier formulations, any departure from uniform motion in a straight line implied the existence of a material impediment to the motion; in the more abstract formulation in the Principia, the existence of an impressed force is implied, with the question of how this force is effected left open.