All systems of philosophy that reject the idea of an intelligent First Cause, which alone is self-existent, and whose being is of a higher order than that of aught else,--base their denial on the assumption that no such distinction of nature either exists or is possible,--that there is but one reality, namely the substance whereof the sensible world consists,--that this has always existed with the same forces it has now, and that it is the source of all phenomena. This assumption of the unreality of whatever is beyond the scope of sense, which has ever been at the bottom of materialistic systems, is now elaborately formulated as a creed, declared by Professor Haeckel and his following to be the only creed which science can tolerate. This is termed _Monism_,--from the Greek Μὁνος, "single," and is opposed to _Dualism_, or the doctrine that there are two orders of being, or two distinct substances, material and spiritual.[164] According to monistic teaching, therefore, there exists but one _Thing_, that which we usually call Matter, but might equally well call Mind,--for all phenomena whatever, whether mental or material, are but various shapes which it assumes, exhibiting diverse aspects of itself. Thus all the objects which appear to have a being of their own,--as the globe we inhabit, the furniture of earth and heaven, we ourselves,--are but the forms momentarily assumed by this protean entity in its ceaseless transfigurations, and have no more existence of their own than the ripples on a pool of water or the faces we see in the fire. It follows that when the particular phase of this basic substance is ended which brings us into being, (or rather which we _are_,) we like everything else, sink into blank nothing,--so that the mighty dead whom nations honour, or the loved ones whose memory we cherish, are blotted out of existence as utterly as the days and nights which made up the span of their lives. But amongst its permutations and combinations this solitary reality can produce the phenomena which we call thought, just as much as those which we call motion, and accordingly the _Aeneid_ or _Hamlet_ is its work, a mechanical product of evolution, no less than a seam of coal, or an eclipse of the moon. Such, in outline, is the philosophical system which commends itself, as Professor Haeckel assures us,[165] to all men of science, who combine the necessary conditions, of scientific knowledge, mental acumen, moral courage, and intellectual independence. It may be rightly described as materialistic pantheism; for while, according to it, everything is equally divine, in the only sense in which anything can be so, everything is likewise equally material, as falling under the category of what we know as matter, and within the direct cognizance of physical science.