(_a_) We have seen (§ 15) that the principles of any science could not rightly be conceived as inscrutable, ultimate, absolute certainties of divine descent, and acknowledging no human ancestry. We saw that they could be understood only as hypotheses which reflection upon a problem had somehow suggested to an ingenious mind, which had been provisionally adopted in order to explore and organize a subject of inquiry, and had finally been verified and confirmed by their success (§ 15 (_c_), § 24). The principles thus accepted by a science are often regarded as descriptive of fact when they are merely methodological and convenient,[401] but this is a point of secondary importance. And even the most amply verified principles never quite lose their hypothetical character. So long as they are used, their meaning, scope, and truth are not absolutely fixed. They can be extended, restricted, and modified by the working of the principles. § 31. (_b_) It is really obvious to any critical reflection that when a science appeals to ‘facts’, it is really appealing to the facts _as known_, or supposed to be known. It cannot from the first presume its knowledge to be absolute, and, _pace_ some of our ‘neo-realists’, ignore the question whether the alleged facts are facts at all, and so pretend to start from ‘the facts as they really are’. Such uncritical temerity would only conduct to insoluble pseudo-problems like that with which King Charles plagued the nascent Royal Society, as to _why_ the weight of a bucket full of water was not increased when a fish was added to it. If, however, it is acknowledged that the ‘facts’ involved in a scientific inquiry are always relative to a definite state and date in the history of a science, several important corollaries follow. (1) Being dependent on the condition of the science, the facts of a science will not all be ‘facts’. That is, not all that is relevant to the interest of the science will actually be within its cognizance, not all that turns out to be fact, and is antedated when it has been discovered, is as yet recognized as fact. It will be this fact, moreover, which constitutes the science a field for inquiry and renders it progressive.