The physical world is now to be regarded as a construction which does not assimilate to itself the content of sensations, but enables one to anticipate them. The sensation signifies a contact to which science can provide a key for practical guidance. [Sidenote: Experimentalism.] § 118. This last phase of naturalism is an attempt to state a pure and consistent experimentalism, a workable theory of the routine of sensations. But it commonly falls into the error of the vicious circle. The hypothetical cause of sensations is said to be matter. From this point of view the sensation is a complex, comprising elaborate physical and physiological processes. But these processes themselves, on the other hand, are said to be analyzable into sensations. Now two such methods of analysis cannot be equally ultimate. If all of reality is finally reducible to sensations, then the term sensation must be used in a new sense to connote a self-subsistent being, and can no longer refer merely to a function of certain physiological processes. The issue of this would be some form of idealism or of the experience-philosophy that is now coming so rapidly to the front.[256:20] But while it is true that idealism has sometimes been intended, and that a radically new philosophy of experience has sometimes been closely approached, those, nevertheless, who have developed experimentalism from the naturalistic stand-point have in reality achieved only a thinly disguised materialism. For _the very ground of their agnosticism is materialistic_.[256:21] Knowledge of reality itself is said to be unattainable, because knowledge, in order to come within the order of nature, must be regarded as reducible to sensation; and because sensation itself, when regarded as a part of nature, is only a physiological process, a special phenomenon, in no way qualified to be knowledge that is true of reality. [Sidenote: Naturalistic Epistemology not Systematic.]