An assumed cause, on the other hand, cannot be accepted as true simply _because_ it explains the phenomena (since two conflicting hypotheses often do this even originally, or, as Dr. Whewell himself allows, may at any rate by modifications be made to do it); nor _because_ it moreover leads to the prediction of other results which turn out true (since this shows only what was indeed apparent already from its agreement with the old facts, viz. that the phenomena are governed by laws partially identical with the laws of other causes); nor _because_ we cannot imagine any other hypothesis which will account for the facts (since there may be causes unknown to our present experience which will equally account for them). The utility of such assumptions _of causes_ depends on their being, in their own nature, _capable_ (as Descartes' Vortices were not, though possibly the Luminiferous Ether may be) of being, at some time or other, proved directly by independent evidence to be the causes. And this was, perhaps, all that Newton meant by his _veræ causæ_, which alone, he said, may be assigned as causes of phenomena. Assumptions of causes, which fulfil this condition, are, in science, even indispensable, with a view both to experimental inquiry, and still more to the application of the Deductive Method. They may be accepted, not indeed, as Dr. Whewell thinks they may be, as proof, but as suggesting a line of experiment and observation which may result in proof. And this is actually the method used by practical men for eliciting the truth from involved statements. They first extemporise, from a few of the particulars, a rude theory of the mode in which the event happened; and then keep altering it to square with the rest of the facts, which they review one by one. The attempting, as in Geology, to conjecture, in conformity with known laws, in what former collocations of known agents (though _not_ known to have been formerly present) individual existing facts may have originated, is not Hypothesis but Induction; for then we do not _suppose_ causes, but legitimately infer from known effects to unknown causes. Of this nature was Laplace's theory, whether weak or not, as to the origin of the earth and planets. CHAPTER XV. PROGRESSIVE EFFECTS, AND CONTINUED ACTION OF CAUSES. Sometimes a complex effect results, not (as has been supposed in the last four chapters) from several, but from _one_ law. The following is the way.