Two approaches to natural kinds

Philosophical treatments of natural kinds are embedded in two distinct projects. I call these the philosophy of science approach and the philosophy of language approach. Each is characterized by its own set of philosophical questions, concerns, and assumptions. The kinds studied in the philosophy of science approach are projectible categories that can ground inductive inferences and scientific explanation. The kinds studied in the philosophy of language approach are the referential objects of a special linguistic category—natural kind terms—thought to refer directly. Philosophers may hope for a unified account addresses both sets of concerns. This paper argues that this cannot be done successfully. No single account can satisfy both the semantic objectives of the philosophy of language approach and the explanatory projects of the philosophy of science approach. After analyzing where the tensions arise, I make recommendations about assumptions and projects that are best abandoned, those that should be retained, and those that should go their separate ways. I also recommend adopting the disambiguating terminology of “scientific kinds” and “natural kinds” for the different notions of kinds developed in these different approaches.

Nouns for Natural Kinds and the Problem of Arbitrariness

This chapter explains the “Kripke-Putnam orthodoxy” about the reference fixing of ordinary natural kind nouns, and some objections to it, especially “arbitrariness problems”: for example, a Kripke-Putnam baptism for “water” doesn’t discriminate between, say, H2O and P2O (H2O where the isotope of hydrogen involved is protium oxide, as in regular paradigms of water). The chapter presents a picture of reference fixing for natural kind nouns that refines the Kripke-Putnam picture and that appeals to sets of roughly sufficient conditions for reference and reference failure. It is argued that on this picture the referents of ordinary natural kind nouns turn out to be “ordinary kinds,” kinds which are vague along dimensions along which scientific kinds are precise: the reference of “water” is “the ordinary kind water” rather than H2O or other scientifically identified kinds. It is argued that this suffices to dispose of the arbitrariness worry on a broadly Kripkean view.

Cancer

Is cancer one or many? If many, how many diseases is cancer, exactly? I argue that this question makes a false assumption; there is no single “natural” classificatory scheme for cancer. Rather, there are many ways to classify cancers, which serve different predictive and explanatory goals. I consider two philosophers’ views concerning whether cancer is a natural kind, that of Khalidi, who argues that cancer is the closest any scientific kind comes to a homeostatic property cluster kind, and that of Lange, whose conclusion is the opposite of Khalidi’s; he argues that cancer is at best a “kludge” and that advances in molecular subtyping of cancer hail the “end of diseases” as natural kinds. I consider several alternative accounts of natural or “scientific” kinds, the “simple causal view,” the “stable property cluster” view, and “scientific kinds,” and argue that the diverse aims of cancer research require us to embrace a much more pluralistic view.