In the preceding considerations, an "equivalence of the Physical and the Psychical" has been assumed, which, though already in a measure defined, should have been, perhaps, more fully explained. It may be repeated that, in such equivalence, no materialistic assumption is made of the dependence of the Psychical on the Physical; nor is the intention to assert that the Psychical can be measured by the weights and measures of the Physical. The assertion is intended in the sense that there is always a physical function connected with the psychical, and that the relation of the two is not an accidental or variable, but a constant one. All that is claimed is, in other words, that, whatever the metaphysical truth as to the freedom of the will, such freedom cannot interfere with the constancy of nature. But, in fact, all that is postulated by physical science in the assertion of the equivalence of physical forces is such a uniformity or constancy of relation as we postulate of the Psychical and Physical; for the different forms of physical force can no more be measured by the same standards than can thought and brain-process.
It may be added, further, that by "force" as used in the above arguments, no metaphysical entity is implied; the word simply serves as the generic term embracing different forms of motion and the equivalent of motion in resistance, and enables us to deal with motion regarded as potential as well as with motion actually existent.
FOOTNOTES:
[96] "Problems of Life and Mind," second series, chap. on Evolution.
[97] "The Variation of Plants and Animals under Domestication," 1868, II. 272.
[98] Vol. II. Chap. XXII.
[99] "Variation of Animals and Plants under Domestication," II. p. 257. See also "Origin of Species," 6th ed., I. pp. 7-9, etc.
[100] "Variation of Animals and Plants under Domestication," II. p. 418.
[101] Ibid.
[102] For elaboration of definition and theory, vide the article in question, "Vierteljahrschrift für wissenschaftliche Philosophie," 1890.