Dissertations and Discussions, vol. i., fourth paper.

Written before the rise of the new views respecting the relation of heat to mechanical force; but confirmed rather than contradicted by them.

As is well remarked by Professor Bain, in the very valuable chapter of his Logic which treats of this subject (ii., 121), “scientific explanation and inductive generalization being the same thing, the limits of Explanation are the limits of Induction,” and “the limits to inductive generalization are the limits to the agreement or community of facts. Induction supposes similarity among phenomena; and when such similarity is discovered, it reduces the phenomena under a common statement. The similarity of terrestrial gravity to celestial attraction enables the two to be expressed as one phenomenon. The similarity between capillary attraction, solution, the operation of cements, etc., leads to their being regarded not as a plurality, but as a unity, a single causative link, the operation of a single agency.... If it be asked whether we can merge gravity itself in some still higher law, the answer must depend upon the facts. Are there any other forces, at present held distinct from gravity, that we may hope to make fraternize with it, so as to join in constituting a higher unity? Gravity is an attractive force; and another great attractive force is cohesion, or the force that binds together the atoms of solid matter. Might we, then, join these two in a still higher unity, expressed under a more comprehensive law? Certainly we might, but not to any advantage.