When organizing the world around us, we divide particular things into kinds of things. Scientists do so as well and theorize about kinds like hydrogen, gold and water; Canis lupus and Felis catus; inflation and democracy; schizophrenia and autism, heterosexual and homosexual.
Given this variety of scientific kind categories, one can ask several philosophical questions. The first question is whether any of these categories are truly natural – rather than conventional – ways of categorizing the world, and what it is that constitutes the difference. Philosophers have proposed many criteria to distinguish natural from conventional categories. In my work on this issue, I have defended several points. Most recently, I have argued that there cannot be a single unified set of criteria that completely accounts for the naturalness of kinds, that a purely epistemic theory cannot work either, and that a successful theory must be a causal one.
The second question one can ask is whether there are only natural kinds in the most fundamental sciences, – like physics and chemistry -, or whether there are also natural kinds in the ‘special’ sciences, like biology, geology, or sociology. One’s answer to the first question about the criteria of natural kinds will also commit one to an answer to this second question. In my work, I have argued that there is no reason to limit the class of natural kinds to the most basic sciences.
The final question is whether particular human kinds also constitute natural kinds. Often the term ‘human kind’ is used for all kinds referred to in the social sciences, including kinds like inflation. As I use the term, however, it applies only to those categories that group people. This includes some categories from the social sciences, like immigrant and refugee, but also kinds from many other sciences like psychiatry (e.g. schizophrenic or autistic) and biology (e.g. human races like white or Asian).
Human kinds present extra worries when evaluating their naturalness. After all, when classifying people, the goal is often not only to represent epistemically useful similarities and differences between individuals. Often, labels for kinds of people are also used to stigmatize and oppress them, or rather to liberate them. I have previously written about the naturalness of such human kinds as sexual orientations and human races.