The object of Kepler was to determine the real path described by each of the planets, or let us say by the planet Mars, (for it was of that body that he first established two of the three great astronomical truths which bear his name.) To do this there was no other mode than that of direct observation: and all which observation could do was to ascertain a great number of the successive places of the planet; or rather, of its apparent places. That the planet occupied successively all these positions, or at all events, positions which produced the same impressions on the eye, and that it passed from one of these to another insensibly, and without any apparent breach of continuity; thus much the senses, with the aid of the proper instruments, could ascertain. What Kepler did more than this, was to find what sort of a curve these different points would make, supposing them to be all joined together. He expressed the whole series of the observed places of Mars by what Dr. Whewell calls the general conception of an ellipse. This operation was far from being as easy as that of the navigator who expressed the series of his observations on successive points of the coast by the general conception of an island. But it is the very same sort of operation; and if the one is not an induction but a description, this must also be true of the other. To avoid misapprehension, we must remark that Kepler, in one respect, performed a real act of induction; namely, in concluding that because the observed places of Mars were correctly represented by points in an imaginary ellipse, therefore Mars would continue to revolve in that same ellipse; and even in concluding that the position of the planet during the time which intervened between two observations, must have coincided with the intermediate points of the curve. But this really inductive operation requires to be carefully distinguished from the mere act of bringing the facts actually observed under a general description. So distinct are these two operations, that the one might have been performed without the other. Men might and did make correct inductions concerning the heavenly motions, before they had obtained correct general descriptions of them. It was known that the planets always moved in the same paths, long before it had been ascertained that those paths were ellipses. Astronomers early remarked that the same set of apparent positions returned periodically. When they obtained a new description of the phenomenon, they did not necessarily make any further induction, nor (which is the true test of a new general truth) add anything to the power of prediction which they already possessed.