The theory of gravitation in astronomy (Newton), the nebular theory in cosmogony (Kant and Laplace), the principle of energy in physics (Meyer and Helmholtz), the atomic theory in chemistry (Dalton), the vibratory theory in optics (Huyghens), the cellular theory in histology (Schleiden and Schwann), and the theory of descent in biology (Lamarck and Darwin), are all important theories of the first rank; they explain a whole world of natural phenomena by the assumption of a common cause for all the several facts of their respective provinces, and by showing that all the phenomena thereof are inter-connected and controlled by laws which issue from this common cause. Yet the cause itself may remain obscure in character, or be merely a "provisional hypothesis." The "force of gravity" in the theory of gravitation and in cosmogony, "energy" itself in its relation to matter, the "ether" of optics and electricity, the "atom" of the chemist, the living "protoplasm" of histology, the "heredity" of the evolutionist--these and similar conceptions of other great theories may be regarded by a sceptical philosophy as "mere hypotheses" and the outcome of scientific "faith," yet they are indispensable for us, until they are replaced by better hypotheses. The dogmas which are used for the explanation of phenomena in the various religions, and which go by the name of "faith" (in the narrower sense), are of a very different character from the forms of scientific faith we have enumerated. The two types, however--the "natural" faith of science and the "supernatural" faith of religion--are not infrequently confounded, so that we must point out their fundamental difference. Religious faith means always belief in a miracle, and as such is in hopeless contradiction with the natural faith of reason. In opposition to reason it postulates supernatural agencies, and, therefore, may be justly called superstition. The essential difference of this superstition from rational faith lies in the fact that it assumes supernatural forces and phenomena, which are unknown and inadmissible to science, and which are the outcome of illusion and fancy; moreover, superstition contradicts the well-known laws of nature, and is therefore _irrational_.