(φυσικός, natural, from φύσις, nature), is, in its most general signification, the science of nature. Modern usage, however, has limited the acceptation of the term to those sciences which treat of body or matter, thus standing distinguished from the mental sciences on the one hand, and from mathematics, or the science of quantity, on the other. Physics, or physical science, thus understood, falls to be divided into the two great heads of Abstract Physics and Concrete Physics. The former is co-extensive with natural philosophy, and treats of mechanical philosophy, including under it mechanics, hydrostatics, and hydraulics, on the one hand; and with chemical philosophy, including under it optics, electricity, and magnetism, on the other. The latter includes natural history, descriptive and philosophical, embracing physiology, geology, and physical astronomy, on the one hand, and concrete chemistry, analytical and explanatory, on the other; as well as zoology, botany, descriptive astronomy, meteorology, mineralogy, and physical geography. Auguste Comte, in his classification of the sciences given in his Philosophie Positive, restricts the term physics to what, in ordinary language, is loosely termed natural philosophy. Physics, according to him, have for their object the discovery of the general laws of the inorganic world. "In physics we study the laws which govern the general properties of bodies ordinarily viewed in their mass, and constantly placed in circumstances capable of maintaining intact the composition of their molecules, and most frequently even their state of aggregation."