measure, and that they only obtain one, if we make them the object of a comparative judgment, i.e. in a sense translate them into the psychological. Disparate values cannot in any way be compared, so long as a transformation of the one into the other is impossible. We can compare warmth and mechanical work, because the one can be transformed into the other according to a strict law of equivalence. But we cannot compare a tone with a sensation of light, or a visual idea with a chord, because a transformation of the one of these practical contents into the other is unthinkable. Now physical values are subject to the principle of the conservation of energy because of the unlimited capacity for transformation of physical energies according to equivalent relations. But it has on the other hand no sense to try to apply this same principle to the qualitative psychical values, which do not in any way admit of such a transformation. This of course stands in close relation with the fact that the subject-matter of psychology is the whole manifoldness of qualitative contents directly presented to our experience, each of which would immediately lose its own peculiar quality, if we tried to transform it into any other. Thus the physical phenomena investigated by the natural sciences and the laws of these phenomena do not in the least contradict the qualitative content of life dealt with by psychology. They rather supplement each other, inasmuch as we must combine them together into one whole, if we wish to understand the life of the psycho-physical being given to us in its unity. Yet this impossibility of comparison of these qualities could not exist along with the unity of their substratum, if the physical and psychical values were not joined together in this substratum. This connection consists herein, that on the one side the physical elements, whether atoms or parts of one continuous matter, must necessarily be thought by us in forms of spatial and temporal ideas arising in accordance with psychical laws, and on the other side the psychical elements, the simple sensations and feelings, are inalienably bound up with definite physical processes. These latter need by no means be of a simple constitution, as has at times been presupposed by reason of metaphysical prejudices. The opposite is rather the case, as experience, which alone in this question can decide, incontestably teaches. For it shows that each simple sensation is joined to a very complicated combination of peripheral and central nerve-processes, and so also with the most elementary feeling, as is shown by the manifold "expression" phenomena which accompany the simplest feeling.