Introduction: natural kinds in general

One of the salient facts about biology that is explained by evolution is the existence of a branching tree of life: a set of categories in which every organism finds its proper place. These placings are not merely matters of human choice or convenience, but are dictated by facts about the organisms themselves. For example, it would be wrong to classify a whale as a fish. This suggests that the biological categories species, genus and so on are prime examples of what are known as natural kinds. Put briefly, natural kinds are categories that are actually there in nature, as opposed to being impositions on nature for our own convenience. It is often said that natural kinds are the categories that are of interest to science. For example, Kripke (1980) holds that it is the business of science to discover natural kinds.

A prime example of a natural kind is a chemical element. All samples of, for example, gold have the same atomic number – 79 – and it is a central pillar of the science of chemistry that elements can be assigned to their proper place in the periodic table of the elements. An alloy is a mixture of two or more metals, but that does not mean that the boundary between one metal and another is vague or fuzzy.