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In philosophy a natural kind is a grouping of things which is a natural grouping, not an artificial one. Or, it is something a set of things (objects, events, beings) has in common which sets it apart from other things as a real set rather than as a group of things arbitrarily lumped together by a person or group of people.

If any natural kinds exist at all, good candidates for being natural kinds might include each of the chemical elements, like gold or potassium. Physical particles like quarks might also be natural kinds. That is, they would still be groups of things, distinct from other things as a group, even if there were no people around to say that they were members of the same group. The set of objects that weigh more than 50 pounds, on the other hand, almost certainly does not comprise a natural kind. A person might group those objects together for some purpose like shipping costs, but there is no particular reason that any other person should lump those objects together instead of placing them in some other grouping.

There is considerable debate in philosophy about whether there are any natural kinds at all, and if so, what they are. Philosophers of biology argue about whether biological species, like "eagle," are natural kinds. Others debate whether races, sexes, or sexual orientations are natural kinds. Meteorologists classify a number of different kinds of clouds, but it is not clear whether they are really different kinds, or whether those groups merely reflect the classifying interests of human beings.

A more formal definition has it that a natural kind is a family of "entities possessing properties bound by natural law; we know of natural kinds in the form of categories of minerals, plants, or animals, and we know that different human cultures classify natural realities that surround them in a completely analogous fashion" (Molino 2000, p.168). The term was brought into contemporary philosophy by W. V. Quine in his essay "Natural Kinds", where any set of objects forms a kind only if (and perhaps if) it is "projectable", meaning judgments made about some members of that set can plausibly be extended by scientific induction to other members. Hence "raven" and "black" are natural kind terms, because any black raven consistutes at least some evidence that all ravens are black. But "nonblack" and "nonraven" are not, because a nonblack nonraven (say, a red herring) is not evidence that all nonblack things are nonravens [1]. Nelson Goodman's problem predicate "grue", meaning "observed before 1 January 2000 and blue or observed after 1 January 2000 and green", turns out to be inappropriate for science because it does not denote a natural kind. Quine argued that kind-hood was logically primitive: it could not be reduced non-trivially to any other relation among individuals.

Cultural artifacts are not generally considered natural kinds. As one author puts it, "they never stop changing, and terms that designate them constitute only what Wittgenstein called 'family resemblance predicates'" (ibid, p.169). This point is more disputed; John McDowell has extensively argued that this opposition between "culture" and "nature" cannot be clearly formulated, and that in any case it ought to lead us to construing cultural products not as unnatural, but as, adopting Aristotle's terminology, a kind of "second nature."

• Molino, Jean (2000). "Toward an Evolutionary Theory of Music and Language", The Origins of Music. Cambridge, Mass: A Bradford Book, The MIT Press. ISBN 0-262-23206-5.

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• natural Kind Terms, an entry from the Encyclopedia of Language and Linguistics, by Steven Gross (PDF).