But though this is sufficient ground for an agnostic policy, it does prove an agnostic theory. The latter has sprung from a closer analysis of knowledge, though it fails to make a very brave showing for thoroughness and consistency. The crucial point has already been brought within our view. The general principles of naturalism require that knowledge shall be reduced to sensations, or impressions of the environment upon the organism. But the environment and the sensations do not correspond. The environment is matter and motion, force and energy; the sensations are of motions, to be sure, but much more conspicuously of colors, sounds, odors, pleasures, and pains. Critically, this may be expressed by saying that since the larger part of sense-perception is so unmistakably subjective, and since all knowledge alike must be derived from this source, knowledge as a whole must be regarded as dealing only with appearances. There are at least three agnostic methods progressing from this point. All agree that the inner or essential reality is unfathomable. But, in the first place, those most close to the tradition of materialism maintain that the most significant appearances, the primary qualities, are those which compose a purely quantitative and corporeal world. The inner essence of things may at any rate be _approached_ by a monism of matter or of energy. This theory is epistemological only to the extent of moderating its claims in the hope of lessening its responsibility. Another agnosticism places all sense qualities on a par, but would regard physics and psychology as complementary reports upon the two distinct series of phenomena in which the underlying reality expresses itself. This theory is epistemological to the extent of granting knowledge, viewed as perception, as good a standing in the universe as that which is accorded to its object. But such a dualism tends almost irresistibly to relapse into materialistic monism, because of the fundamental place of physical conceptions in the system of the sciences. Finally, in another and a more radical phase of agnosticism, we find an attempt to make full provision for the legitimate problems of epistemology. The only datum, the only existent accessible to knowledge, is said to be the sensation, or state of consciousness. In the words of Huxley: "What, after all, do we know of this terrible 'matter' except as a name for the unknown and hypothetical cause of states of our own consciousness? And what do we know of that 'spirit' over whose threatened extinction by matter a great lamentation is arising, . . . except that it is also a name for an unknown and hypothetical cause, or condition, of states of consciousness?"[255:19]