Paul Needham

Chemistry builds on distinctions of substance, which presupposes that matter can be divided into substances and compared with other matter and itself on different occasions as being of the same substance. Even identifying a quantity of matter as comprising a single substance presupposes the same substance relation, it being a quantity all of whose spatial parts are the same substance. But 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be compared with microscopic conceptions of molecular structure, with which they have been complemented—some would say superseded.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.

Prior left us with a problem which he stated in the following way: ‘Very roughly, it would seem that countable “things” are made or grow from bits of stuff, or from other countable “things”, that are already there. The precise logic of this process hasn’t been worked out yet, and until it has been, it seems likely that any tensed predicate logic can only be provisional in character.’ Although I disagree with much of the philosophy of time underlying Priorean tense logic, the problem of presenting a coherent view of ‘countable things’ and ‘bits of stuff’ within a framework accommodating temporal reference and modal expressions along broadly the lines Prior proposed remains an interesting one. The present paper sketches a general framework within which more specific questions raised by this general issue can be addressed, and makes a number of tentative suggestions without, however, proceeding so far as to settle on and develop a definite logical theory.

Chemistry, the science of substances and their transformations with roots in antiquity, provides as rich a source as any of the claims about what is not directly observable in the light of ideas reflecting both constancy and change. An important distinction in chemistry is that between macroscopic and microscopic realms, mistaken by positivists as a distinction between observable and theoretical and later by certain realists as a distinction between the merely superficial and the deeply theoretical. Recent realist themes about the preservation of the extension of substance predicates typified by “water” and the view that 20th century scientific atomism vindicates earlier philosophical speculations about the corpuscular constitution of matter are unjustified. The relation between quantum and general chemistry is vexed. Despite questioning particular realist theses, however, none of this raises doubts about the basic realist stance that science seeks and sometimes achieves knowledge of a world independent of our thoughts.

This article surveys the theory of the part relation (mereology), quantified modal logic, and Kripke and Putnam’s notion of natural kinds. It shows how the former two bear on the macroscopic understanding of the notions of substance and phase, which stands in contrast to the microphysical essentialism of Kripke and Putnam, and can be used to explicate Aristotle’s and the Stoic conceptions of mixture. The article concludes with some comments about the relevance of the issues raised by these ancient theories to modern views of element, compound and mixture.

Despite the importance molecular structure has acquired in 20th century chemistry, more traditional macroscopic notions in terms of a continuous concept of matter continue to play a role in chemical theorising. In the light of the extensive and determined criticism of reductionism in recent philosophy of chemistry, it is of interest to see macroscopic ontology treated autonomously. One aspect of this is developed here, namely the concept of chemical substance. This is characterised by contrast with phases and solutions. The key conception is that of an intensive property, which is defined by appeal to the mereological structure of parts of the entities bearing substance properties.