How can a law become a principle? It expressed a relation between two real terms _A_ and _B_. But it was not rigorously true, it was only approximate. We introduce arbitrarily an intermediary term _C_ more or less fictitious, and _C_ is _by definition_ that which has with _A_ _exactly_ the relation expressed by the law. Then our law is separated into an absolute and rigorous principle which expresses the relation of _A_ to _C_ and an experimental law, approximate and subject to revision, which expresses the relation of _C_ to _B_. It is clear that, however far this partition is pushed, some laws will always be left remaining. We go to enter now the domain of laws properly so called. PART IV NATURE CHAPTER IX HYPOTHESES IN PHYSICS THE RÔLE OF EXPERIMENT AND GENERALIZATION.--Experiment is the sole source of truth. It alone can teach us anything new; it alone can give us certainty. These are two points that can not be questioned. But then, if experiment is everything, what place will remain for mathematical physics? What has experimental physics to do with such an aid, one which seems useless and perhaps even dangerous? And yet mathematical physics exists, and has done unquestionable service. We have here a fact that must be explained. The explanation is that merely to observe is not enough. We must use our observations, and to do that we must generalize. This is what men always have done; only as the memory of past errors has made them more and more careful, they have observed more and more, and generalized less and less. Every age has ridiculed the one before it, and accused it of having generalized too quickly and too naïvely. Descartes pitied the Ionians; Descartes, in his turn, makes us smile. No doubt our children will some day laugh at us. But can we not then pass over immediately to the goal? Is not this the means of escaping the ridicule that we foresee? Can we not be content with just the bare experiment? No, that is impossible; it would be to mistake utterly the true nature of science. The scientist must set in order. Science is built up with facts, as a house is with stones. But a collection of facts is no more a science than a heap of stones is a house. And above all the scientist must foresee. Carlyle has somewhere said something like this: "Nothing but facts are of importance. John Lackland passed by here. Here is something that is admirable. Here is a reality for which I would give all the theories in the world." Carlyle was a fellow countryman of Bacon; but Bacon would not have said that. That is the language of the historian. The physicist would say rather: "John Lackland passed by here; that makes no difference to me, for he never will pass this way again."