Natural kinds

2. Two major questions

The topic of natural kinds raises two major related questions. There is a metaphysical question: what is a natural kind? Natural kinds are standardly distinguished from arbitrary groups of objects, such as what you had for breakfast. They are also standardly distinguished from so-called artefact kinds such as books, tables or cars. A full account of the metaphysics of natural kinds should clarify these distinctions and explain the rationale behind them. Furthermore, it should explain the sense in which different members of a natural kind have a property in common. On one possible account, different members of a natural kind share a universal: they have a property which is literally identical between them (see Universals). But on a rival account, different members of a kind have a property in common only in the sense that each member bears a certain similarity relation to every other member of the kind.

There is also an epistemological question: how do we tell what natural kinds there are? Every object has many properties, yet we think that only some properties determine natural kinds. But what reasons guide our thinking that certain properties rather than others determine natural kinds? Furthermore, our beliefs about what natural kinds there are, and which objects belong to them, may be inaccurate to some degree. These beliefs may only approximately describe what natural kinds there are, and what their members are. But then how are we to establish how accurate these beliefs are?

Such metaphysical and epistemological questions are also of interest outside philosophy. For instance, it is a live issue among biologists whether species are natural kinds, and there is a further issue of whether it is morphological, reproductive or phylogenetic factors which determine a creature’s membership of a given species (see Species).