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ideas, there still remains a distinction between ideas of material movement and ideas of phenomena of consciousness; and thus again arises the problem how these different sets of ideas, which have arisen in accordance with experience, are to be combined. The problem for psychology is empirical, and is independent of the metaphysical. We do not ask whether mind or matter is most fundamental; we inquire in what way mental and material phenomena are connected in that experience which every system of metaphysics consciously or unconsciously presupposes.

The fourth possibility, the identity hypothesis, regards the mental and the material worlds as two manifestations of one and the same being, both given in experience.

If it is contrary to the doctrine of persistence of force to suppose a transition from the one province to the other, and if nevertheless the two provinces exist in our experience as distinct, then the two sets of phenomena must be unfolded simultaneously, each according to its own laws; so that for every phenomenon in the world of consciousness there is a corresponding phenomenon in the world of matter, and conversely. The parallels already drawn point directly to such a relation; and it would be an amazing accident if, while the characteristic marks repeated themselves in this way, there were not at the foundation an inner connection. Both the parallelism and proportionality between the activity of consciousness and cerebral activity point to an identity at bottom. The difference which remains in spite of the points of agreement compels us to suppose that one and the same principle has found its expression in a double form. We have no right to take mind and body for two beings in reciprocal interaction. We are, on the contrary, impelled to conceive the material interaction between the elements composing the brain and nervous system as an outer form of the inner ideal unity of consciousness. What we in our inner experience become conscious of as thought, feeling, and resolution, is thus represented in the material world by certain material processes of the brain, which as such are subject to the law of the persistence of energy, although this law can not be applied to the relation between cerebral and conscious processes. It is as though the same thing were said in two languages. . ..

In the mental as in the material world we hold fast the law of continuity. The identity hypothesis regards both these worlds as two manifestations of one and the same being, both given in experience.

The two languages in which the same thought is here expressed, we are not able to trace back to a common original language. So long as we keep strictly to experience, one province is presented as a fragment while the other extends to infinity. The doctrine of the persistence of energy makes the material world into a totality which we can never measure, but in which the fate of the individual forms and elements can be traced. The mental world has no corresponding law to exhibit. Mental elements come and go in experience without our being able to point to an equivalent. The fact that mental states can not be measured like physical energies and chemical substances is, in itself, sufficient to frustrate the hope of our finding a mental parallel to the doctrine of the persistence of force. But, in addition to this, even the fundamental conception of a mental existence puts difficulties in the way. Material existences can pass one into another, so that the energy lost in one is preserved in the other. The doctrine of the persistence of energy shows us the unity and eternity of Nature during the coming and going of all material beings; but mental existence, as has been seen, has for its