Now science, as we have just seen, is tolerant of this notion of latency or potentiality, and is ready enough to use it in aid of beliefs in conservation. It was so used in connection with heat when heat was regarded as a material substance. It is still so used in connection with energy, which is sometimes described as an immaterial substance. But (as I have already noted) it has never been so used in connection with matter. The reason, I suppose, is that the conservation of matter is much more a belief of common sense than the conservation of energy. Energy is a conception which has but recently been disengaged from other conceptions, like force and momentum, and has but recently been associated with heat, with chemical reactions, with changes of physical phase, and with electro-magnetic phenomena. It is, therefore, a remote and somewhat abstract product of scientific reflection; and science may do what it will with its own. The notion of matter, on the other hand, is the common possession of mankind. Whatever difficulties it may present to reflective analysis, it presents none to our work-a-day beliefs. We are quite ready to regard it as indestructible; but we are not ready to combine this conviction with the view that it possesses no single characteristic which may not be temporarily etherealised into a “potentiality.” On such terms the eternal and unchanging identity of this or that parcel of matter would seem a difficult and elusive doctrine, inappropriate to the familiar and substantial world in which we suppose ourselves to live. A belief in the conservation of matter has therefore always, or almost always, carried with it a belief in the unchanging continuity of at least _some_ material qualities; though as to what these qualities are there has been much dispute. Descartes, though not consistent, found unchanging continuity in the attribute of size; so also did Hobbes. I presume that the older atomists, who explained the appearances of matter by the shape of its constituent atoms, would have regarded both atomic form and atomic magnitude as persistent. But it was the assumption that the same piece of ponderable matter always possessed the same gravitating power, and that the same gravitating power was always associated with the same mass, which, in the hands of Lavoisier, made so great a revolution in eighteenth-century chemistry. Matter might change its size, its shape, its colour, its phase, its power of acting and reacting; but its mass and the quality which caused its weight it could not change; these characteristics were always associated with each other, and were never in abeyance.