I have said that in this case the verification fulfils the conditions of an induction: but an induction of what sort? On examination we find that it conforms to the canon of the Method of Difference. It affords the two instances, A B C, _a b c_, and B C, _b c_. A represents central force; A B C, the planets _plus_ a central force; B C, the planets apart from a central force. The planets with a central force give _a_, areas proportional to the times; the planets without a central force give _b c_ (a set of motions) without _a_, or with something else instead of _a_. This is the Method of Difference in all its strictness. It is true, the two instances which the method requires are obtained in this case, not by experiment, but by a prior deduction. But that is of no consequence. It is immaterial what is the nature of the evidence from which we derive the assurance that A B C will produce _a b c_, and B C only _b c_; it is enough that we have that assurance. In the present case, a process of reasoning furnished Newton with the very instances, which, if the nature of the case had admitted of it, he would have sought by experiment. It is thus perfectly possible, and indeed is a very common occurrence, that what was an hypothesis at the beginning of the inquiry, becomes a proved law of nature before its close. But in order that this should happen, we must be able, either by deduction or experiment, to obtain _both_ the instances which the Method of Difference requires. That we are able from the hypothesis to deduce the known facts, gives only the affirmative instance, A B C, _a b c_. It is equally necessary that we should be able to obtain, as Newton did, the negative instance B C, _b c_; by showing that no antecedent, except the one assumed in the hypothesis, would in conjunction with B C produce _a_.