214 INDUCTION. 



had observed the whole of what the pi'oposition asserts. That the land in 

 question is an island, is not an inference from the partial facts which the 

 navigator saw in the course of his circumnavigation ; it is the facts them- 

 selves; it is a summary of those facts; the description of a complex fact, 

 to which those simpler ones are as the parts of a whole. 



Now there is, I conceive, no difference in kind between this simple op- 

 eration, and that by which Kepler ascertained the nature of the planetary 

 orbits : and Kepler's operation, all at least that was characteristic in it, was 

 not more an inductive act than that of our supposed navigator. 



The object of Kepler was to determine the real path described by each 

 of the planets, or let us say by the planet Mai's (since it was of that body 

 that he first established the two of his three laws which did not require a 

 comparison of planets). 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 ap- 

 parent 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 lind what sort of a curve these different points would make, supposing 

 them to be all joined together. He expressed the whole series of the ob- 

 served 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 navi- 

 gator 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. 



The only real induction concerned in the case, consisted in inferring 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 in concluding (before the gap had been filled up by fur- 

 ther observations) that the positions of the planet during the time which 

 intervened between two obsei'vations, must have coincided with the inter- 

 mediate points of the curve. For these were facts which had not been di- 

 rectly observed. They were inferences from the observations; facts in- 

 ferred, as distinguished from facts seen. But these inferences were so far 

 from being a part of Kepler's philosophical operation, that they had been 

 drawn long before he was born. Astronomers had long known that the 

 planets periodically returned to the same places. When this had been as- 

 certained, there was no induction left for Kepler to make, nor did he make 

 any further induction. He merely applied his new conception to the facts 

 inferred, as he did to the facts observed. Knowing already that the plan- 

 ets continued to move in the same paths ; when he found that an ellipse 

 correctly represented the past path, he knew that it would represent the 

 future path. In finding a compendious expression for the one set of facts, 

 he found one for the other : but he found the expression only, not the in- 

 ference; nor did he (which is the true test of a general truth) add any 

 thing to the power of prediction already possessed. 



§ 4. The descriptive operation which enables a number of details to be 

 summed up in a single proposition. Dr. Whewell, by an aptly chosen ex- 

 pression, has termed the Colligation of Facts. In most of his observations 



