Natural Kinds has become an important topic among metaphysics, philosophy of language and philosophy of science. What are natural kinds? There have been various theories and views which aim to account for natural kinds, such as natural kind essentialism, HPC, natural kind pluralism, epistemology-only theories of natural kinds, etc.

Other philosophers focus on what the meaning of natural kind terms are. The descriptivism, which is supported by Locke, Frege and Russell, and causal-historical theory, which is proposed by Kripke and Putnam, are disputing with each other, as they provide different views on the reference of natural kind terms.

However, philosophers of science pay more attention to classifications in scientific practice, and a lot of categories in special sciences pose serious challenges to traditional theories of natural kinds. For instance, there are many debates over whether biological species and social categories are natural kinds. So, a unified theory of natural kinds may still be necessary, which is not only explaining the metaphysical nature of natural kinds, but also accommodating natural kind classification in scientific practice.