Over all his special laws of evolution there reigns an absolutely general law, that of the "persistence of force." By this Spencer sometimes means the phenomenal law of conservation of energy, sometimes the metaphysical principle that the quantity of existence is unalterable, sometimes the logical principle that nothing can happen without a reason, sometimes the practical postulate that in the absence of any assignable difference you must call a thing the same. This law is one vast vagueness, of which I can give no clear account; but of his special vaguenesses "mental force" and "social force" are good examples. These manifestations of the universal force, he says, are due to vital force, and this latter is due to physical force, both being proportionate to the amount of physical force which is "transformed" into them. But what on earth is "social force"? Sometimes he identifies it with "social activity" (showing the latter to be proportionate to the amount of food eaten), sometimes with the work done by human beings and their steam-engines, and shows it to be due ultimately to the sun's heat. It would never occur to a reader of his pages that a social force proper might be anything that acted as a stimulus of social change,--a leader, for example, a discovery, a book, a new idea, or a national insult; and that the greatest of "forces" of this kind need embody no more "physical force" than the smallest. The measure of greatness here is the effect produced on the environment, not a quantity antecedently absorbed from physical nature. Mr. Spencer himself is a great social force; but he ate no more than an average man, and his body, if cremated, would disengage no more energy. The effects he exerts are of no nature _of releases_,--his words pull triggers in certain kinds of brain. The fundamental distinction in mechanics between forces of push-and-pull and forces of release is one of which Mr. Spencer, in his earlier years, made no use whatever. Only in his sixth edition did he show that it had seriously arrested his attention. In biology, psychology, and sociology the forces concerned are almost exclusively forces of release. Spencer's account of social forces is neither good sociology nor good mechanics. His feeble grasp of the conception of force vitiates, in fact, all his work.