The extensive division of labor which has taken place in science, on account of the enormous growth of our knowledge in the nineteenth century and the rise of many new disciplines, has very much altered their relations to each other and to the whole, and has even given a fresh meaning and connotation to the term. Hence by physics, as it is now taught at the universities, is usually understood only that part of inorganic science which deals with the molecular relations of substance and the mechanism of mass and ether, without regard to the qualitative differences of the elements, which are expressed in the atomic weight of their smallest particles, the atoms. The study of the atoms and their affinities and combinations belongs to chemistry. As this province is very extensive and has its special methods of research, it is usually put side by side with physics as of equal importance; in reality, however, it is only a branch of physics--chemistry is the physics of the atoms. Hence, when we speak of a physico-chemical inquiry or phenomenon, we might justly describe it briefly as _physical_ (in the wider sense). Physiology, again, a particularly important branch of it, is in this sense the physics of living things, or the physico-chemical study of the living body. Since Aristotle dealt with the eternal phenomena of nature in the first part of his works, and called this _physics_, and with their inner nature in the second part, to which he gave the name of _metaphysics_, the two terms have undergone many and considerable modifications. If we restrict the term "physics" to the empirical study of phenomena (by observation and experiment), we may give the name of metaphysics to every hypothesis and theory that is introduced to fill up the gaps in it. In this sense the indispensable theories of physics (such as the assumption that matter is made up of molecules and atoms and electrons) may be described as metaphysical; such also is our assumption that all substance is endowed with sensation as well as extension (matter). This monistic metaphysics, which recognizes the absolute dominion of the law of substance in all phenomena, but confines itself to the study of nature and abandons inquiry into the supernatural, is, with all its theories and hypotheses, an indispensable part of any rational philosophy of life. To claim, as Ostwald does, that science must be free from hypotheses is to deprive it of its foundations. But it is very different with the current dualistic metaphysics, which holds that there are two distinct worlds, and which we find in a hundred forms as philosophic dualism.