In the philosophy of science and some other branches of philosophy, a "natural kind" is an intellectual grouping, or categorizing of things, that is reflective of the actual world and not just human interests. Some treat it as a classification identifying some structure of truth and reality that exists whether or not humans recognize it. Others treat it as intrinsically useful to the human mind, but not necessarily reflective of something more objective. Candidate examples of natural kinds are found in all the sciences, but the field of chemistry provides the paradigm example of elements. Alexander Bird and Emma Tobin see natural kinds as relevant to metaphysics, epistemology, and the philosophy of language, as well as the philosophy of science.