We organize the world around us by categorizing particular things into kinds. Scientists do this too, theorizing about kinds such as hydrogen, gold and water; Canis lupus and Felis catus; inflation and democracy; schizophrenia and autism, heterosexual and homosexual. This practice of categorizing things into kinds prompts several philosophical questions.
The first question is whether any of these categories are truly natural rather than conventional and what constitutes the difference. To address this, a number of criteria for ‘naturalness’ have been suggested by philosophers. In my research, I defend the view that a successful theory must at least partially rely on causal criteria, and that a theory based on only epistemic criteria cannot be successful.
A second question is whether there are only natural kinds in the most fundamental sciences, such as physics and chemistry, or whether there are also natural kinds in the ‘special’ sciences, like biology, geology, or sociology. 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 human kinds can also be natural kinds. In my use of the term, ‘human kind’ applies to all those categories that group humans. This includes some categories from the social sciences (e.g., immigrant and refugee) but also kinds from other sciences like psychiatry (e.g. schizophrenic or autistic) and biology (e.g. human races like white or Asian).
Evaluating the naturalness/conventionality of different human kinds presents additional challenges. When classifying people, the goal is often not only to identify scientifically useful similarities and differences between individuals. Labels for kinds of people can also be used to oppress or liberate them. It is from this perspective that I have done work evaluating the naturalness of such human kinds as psychiatric disorders, sexual orientations, and human races.