Natural Kinds in Chemistry

Abstract

Chemical substances such as gold and water provide paradigm examples of natural kinds: They are so central to philosophical discussions on the topic that they often provide the grounds for quite general philosophical claims—in particular that natural kinds must be hierarchical, discrete, and independent of interests. In this chapter I will argue that chemistry in fact undermines such claims. In what follows I will (i) introduce the main kinds of chemical kinds, namely chemical substances and microstructural species; (ii) critically examine some general criteria for being a natural kind in the light of how they apply to chemical kinds; and finally (iii) present two broad theories of how chemical substances are individuated. The primary purpose of this article is to bring scientific detail and sophistication to a topic—natural kinds—which has a long but not always honorable history in philosophy, but chemists can also learn something from these discussions. Chemistry is in the business of making general claims about substances, a fact which is embodied in the periodic table, as well as in the systems of nomenclature and classification published by the International Union of Pure and Applied Chemistry (IUPAC). At several points in the history of their subject, chemists appear to have faced choices about which general categories should appear in these systems. Understanding why these choices were made, and the alternatives rejected, gives us an insight into whether chemistry might have developed differently. This is central to understanding why chemistry looks the way it does today. So, what are the chemical kinds? Chemists study the structure and behavior of substances such as gold, water and benzene, and also of microscopic species such as gold atoms, and water and benzene molecules. They group together higher kinds of substances: groups of elements such as the halogens and alkali metals, broader groups of elements such as the metals, and classes of compounds that share either an elemental component (e.g., chlorides), a microstructural feature (e.g., carboxylic acids), or merely a pattern of chemical reactivity (e.g., acids).