That resemblance cannot be physical or spatial, because mental phenomena do not occupy space, or possess weight, or exist in three dimensions. As far as one can see, it consists in two points: both classes of phenomena (1) are objects of thought and (2) are displays of force or power. This implies that movements in space are displays of the same force as is manifested in the non-spatial movements of thought. The force which is displayed in consciousness as Will appears in space as motion. Both classes of phenomena, however, are not only displays of force, but also objects of thought. The reality of a phenomenon of either class consists in its being the manifestation of Will to or in consciousness. The reality of the two classes is co-extensive with their similarity, and is the sole foundation for any true inferences or justifiable generalisations about them.
If the law of "the instability of the homogeneous" is a first principle, and is a scientific induction based upon a real similarity between the mental and material phenomena of which it is offered as an explanation, it becomes interesting to inquire how far the similarity extends. In the case of mental evolution the essential feature of the process is that the mind gradually comes to perceive resemblances and to discriminate differences which, though they were present all the time, were not at first appreciable. In other words, the apparently homogeneous was, from the beginning, really heterogeneous. Now this fact, which is true in the sphere of mental evolution, finds its exact parallel in the evolution of the material universe, if the account given above of mechanical compounds and chemical combinations be true. What appears first in the process of evolution, and is exemplified by mechanics, is matter apparently inert. But when the particles of this apparently inert matter enter into chemical combinations with one another they reveal a fresh set of properties, quite different from those exhibited by them previous to their combination; and, when they enter into physiological relations, they display yet further additional properties. This progressive manifestation it is, and not the accompanying "dissipation of motion and integration of matter," which constitutes evolution in the material world, and finds its exact parallel in mental evolution. In both cases we have Will manifested as object of thought; and in both cases we judge most truly of that which is manifested when we judge it by its most complete manifestation. In both cases the apparent homogeneity is not the ultimate fact underlying everything, but is only the first-fruits of that which is yet to come.
FOOTNOTES:
[8] First Principles, § 46.
[9] Mill, Inductive Logic, bk. iii. ch. xii. p. 549.
[10] Ibid.
[11] First Principles, ch. iii. § 46.
[12] Seth, Chambers's Encyclopædia, s.v. "Philosophy."
[13] Kirchoff, Vorlesungen über mathematische Physik, i. p. 1.
[14] Mertz, History of European Thought in the Nineteenth Century, i. pp. 382, 3, note.