Paul Needham

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.

Classical atoms—“part-less, ontologically irreducible simples” as the conference flyer puts it—are not the atoms of modern chemistry and analogies with the latter can be construed in various ways. They have figured in the historical development of concepts of chemical affinity but without, as Alan Chalmers and I have independently argued, making any significant contribution to empirically justified theories. A purely combinatorial conception of the formation of compounds by juxtaposing atoms is associated with Daltonian atomism. I review the merits of this idea as a solution to problems posed by developments in early nineteenth century chemistry and go on to suggest that subsequent developments in chemistry are as much in agreement with ideas associated with Aristotle as with the ancient atomists, if not more so. The basic issues can be understood without getting involved in detailed chemistry and I hope what I have to say will be of interest to philosophers concerned with the metaphysics of matter but with no special knowledge of chemistry.

From a modern point of view, compounds are contrasted with elements of which they are composed, and the two categories combine to give the category of substances. Mixtures, on the other hand, might be understood to contrast with pure substances (substances in isolation), so that mixtures are quantities of matter containing several substances (be they compounds or uncombined elements) whereas pure substances are understood to be quantities of matter exhausting the material contents of a region of space which contain only one substance, and might be either a compound or an element. But heterogeneous quantities of matter (comprising several phases) might also be understood to be mixtures. Thus, it is common to speak of a mixture of ice, liquid water and water vapour in equilibrium at the triple point of water, although there is only a single substance present. The general concept of mixture as treated by Gibbs’ phase rule is a quantity of matter comprising either several substances or several phases, “or” being used in the inclusive sense. Gibbs’ phase rule is a relatively recent development in the history of the concept of chemical substance, dating from the latter half of the nineteenth century. The concepts and distinctions it presupposes have not always been apparent. I trace some of the important conceptual developments underlying the important distinctions at issue here from Aristotle, who didn’t distinguish either substance and phase, or what are now called compounds and solutions.

The Aristotelian notion of a proper mixture is that of a homogeneous body potentially separable into a definite proportion of elements. Its relation to more modern chemical ideas is not without interest despite the success of modern atomic theory. But there is a fundamental conflict entailed by Aristotle’s two approaches to the characterisation of elements, one in terms of the properties they exhibit in isolation and another in terms of their role as constituents of compounds. Although one source of problems for Aristotle has been put aside in modern chemistry, it seems that the underlying tension remains an issue.