Independently of Mayer, however, the principle was reached almost at the same time by the famous physiologist, Hermann Helmholtz; five years afterwards he pointed out its general application to, and fertility in, every branch of physics. We ought to say to-day that it rules also in the entire province of physiology--that is, of "organic physics"; but on that point we meet a strenuous opposition from the vitalistic biologists and the dualist and spiritualist philosophers. For these the peculiar "spiritual forces" of human nature are a group of "free" forces, not subject to the law of energy; the idea is closely connected with the dogma of the "freedom of the will." We have, however, already seen (p. 204) that the dogma is untenable. Modern physics draws a distinction between "force" and "energy," but our general observations so far have not needed a reference to it. The conviction that these two great cosmic theorems, the chemical law of the persistence of matter and the physical law of the persistence of force, are fundamentally one, is of the utmost importance in our monistic system. The two theories are just as intimately united as their objects--matter and force or energy. Indeed, this fundamental unity of the two laws is self-evident to many monistic scientists and philosophers, since they merely relate to two different aspects of one and the same object, the _cosmos_. But, however natural the thought may be, it is still very far from being generally accepted. It is stoutly contested by the entire dualistic philosophy, vitalistic biology, and parallelistic psychology; even, in fact, by a few (inconsistent) monists, who think they find a check to it in "consciousness," in the higher mental activity of man, or in other phenomena of our "free mental life."