One Substance or More?

Abstract

Chemistry builds on distinctions of substance, which presupposes that matter can be divided into substances and these exemplars compared with others on different occasions to determine whether they are the same substance. Even the notion of a quantity comprising a single substance presupposes the same substance relation, i.e. being a quantity all of whose spatial parts are the same substance. Criteria of purity have been important for isolating substances and investigating their characteristic properties, which can in turn be used for establishing sameness of substance. With the development of chemistry into a theoretical science it became important that such criteria and characteristics should have a systematic theoretical basis. Thermodynamics was, perhaps, the first comprehensive theory to systematically divide the mass of the bodies with which it deals into distinct substances and offer general criteria governing the number of substances present. But the applicability of such macroscopic criteria is restricted to equilibrium conditions on a macroscopic time scale. They can be compared with microscopic conceptions of molecular structure, with which they have been complemented—some would say superseded. Since many substances are not molecular, however, the general formulation of a microscopic sameness of substance criterion remains unclear.