nificance as theory of matter. For the theory of energy the content of experience is given as a state which consists in a distribution of qualities in space and a change of the same in time. The general character of every content of experience always appears as a combination in which one can reflect upon either the moment of unity or the moment of multiplicity. As moment of unity there is found in every object the form of determination according to law, as moment of multiplicity the coördination and succession of space and time. Objects of nature are states of space and time determined according to law, and only of these does the theory of energy treat. The forms of determination according to law which as determining unities are constitutive of objects, are called categories. Two sorts of categories, quantity and quality, are discovered according as we distinguish the states of things with respect to multiplicity or with respect to the unities which condition through their combination the states themselves. By quantity and quality objects of nature, i.e., distribution and change of states in space and time, are fully determinable. Each object is represented as a magnitude under the form of unity, while the plurality of unities of the same sort gives a higher unity so that a whole arises. The essence of the whole consists in the fact that it contains all the unities of its parts, no more and no less. Hence the three categories of quantity: Unity, Plurality, Totality. Every object in Physics may be represented as a numerical distribution of magnitudes in space and time. The concept of energy, however, involves beyond this the problem of the representation of qualities as magnitudes. The theory of energy rests on the possibility of representing change as magnitude. Corresponding to the quantitative categories of Unity, Plurality and Totality are the quantitative categories of Identity, Diversity and Variability which are involved in the concept of change.
While these categories of quantity and quality suffice to define objects of nature, they do not determine whether these objects really exist for experience. It is essential to actual objects of experience that they have independent persistence in space and time, that they exercise effect upon one another, and that in these effects our own bodies be included. The law conditioning these characteristics is the concept of relation. We must therefore expect to find in the concept of energy the categories of relation. The physical object is distinguished from the same object as merely thought by the possession of energy. The first characteristic property of energy is that amid all changes it persists. Hence the principle of the conservation
