INDUCTIONS IMPROPERLY SO CALLED. 325 



The object of Kepler was to determine the real path de- 

 scrihed hy each of the planets, or let us say by the planet 

 Mars (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 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 dif- 

 ferent 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 concep- 

 tion of an ellipse. This operation was far from being as easy 

 as that of the navigator who expressed the series of his obser- 

 vations on successive points of the coast by the general con- 

 ception 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 cor- 

 rectly 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 further obser- 

 vations) that the positions of the planet during the time which 

 intervened between two observations, must have coincided 

 with the intermediate points of the curve. For these were 

 facts which had not been directly observed. They were 

 inferences from the observations; facts inferred, as distin- 

 guished 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 ascertained, there was no 



