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INDUCTION. 



was something added to the facts. He expresses himself as 

 if Kepler had put something into the facts by his mode of 

 conceiving them. But Kepler did no such thing. The ellipse 

 was in the facts before Kepler recognised it ; just as the island 

 was an island before it had been sailed round. Kepler did not 

 put what he had conceived into the facts, but saw it in them. 

 A conception implies, and corresponds to, something conceived: 

 and though the conception itself is not in the facts, but in our 

 mind, yet if it is to convey any knowledge relating to them, 

 it must be a conception of something which really is in the 

 facts, some property which they actually possess, and which 

 they would manifest to our senses, if our senses were able to 

 take cognizance of it. If, for instance, the planet left behind 

 it in space a visible track, and if the observer were in a fixed 

 position at such a distance from the plane of the orbit as 

 would enable him to see the whole of it at once, he would see 

 it to be an ellipse ; and if gifted with appropriate instruments 

 and powers of locomotion, he could prove it to be such by 

 measuring its different dimensions. Nay, further: if the 

 track were visible, and he were so placed that he could see all 

 parts of it in succession, but not all of them at once, he might 

 be able, by piecing together his successive observations, to 

 discover both that it was an ellipse and that the planet moved 

 in it. The case would then exactly resemble that of the navi- 

 gator who discovers the land to be an island by sailing round 

 it. If the path was visible, no one I think would dispute that 

 to identify it with an ellipse is to describe it : and I cannot see 

 why any difference should be made by its not being directly 

 an object of sense, when every point in it is as exactly ascer- 

 tained as if it were so. 



Subject to the indispensable condition which has just 

 been stated, I cannot conceive that the part which concep- 

 tions have in the operation of studying facts, has ever been 

 overlooked or undervalued. No one ever disputed that in 

 order to reason about anything we must have a conception 

 of it ; or that when we include a multitude of things under a 

 general expression, there is implied in the expression a 

 conception of something common to those things. But it 



