But now the question arises: does the search after these agencies henceforth remain the exclusive task of natural science, and have we therefore simply to wait and see whether it will succeed in finding them? or have we to look for the answer to these questions, which natural science can no longer give, in another science--namely, philosophy? The first question we will have to answer in the affirmative, the second in the negative. It is certainly understood that _metaphysical_ principles must underlie all _physical_ appearances; and the right to define these principles, so far as they can be known, is willingly conceded to philosophy by the scientists, with the exception of those of materialistic and naturalistic tendencies. This mutual re-approaching of philosophy and natural science is one of the most gratifying, and, to both, most fruitful evidences of the intellectual work of the present generation. But these metaphysical principles themselves become cognizable only when the physical effects, whose cause they are, become accessible to our knowledge; and every attempt to find them _a priori_, or only to extend them _a priori_, will always fail through the opposition of empirical facts; or even if this attempt accommodates itself to the existing state of knowledge at a given time, it will always be overcome by the {110} progress of the empirical sciences. In the most favorable case, it can claim the value of a hypothesis which has to be put to the proof, whether it can be empirically confirmed and whether we can successfully operate with it in knowing the world of realities. But herewith it leaves the realm of pure philosophy, and makes the question of its right to exist dependent upon the decision of natural science.