Natural kind

In analytic philosophy, the term natural kind is used to refer to a "natural" grouping, not an artificial one. Or, it is something that a set of things (objects, events, beings) has in common which distinguishes it 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 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 constitute 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.

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 analytic 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 "projectible", meaning judgments made about some members of that set can plausibly be extended by scientific induction to other members. Hence "raven" and "black" refer to concepts that match natural kinds because any black raven constitutes at least some evidence that all ravens are black. But "nonblack" and "nonraven" are not, because a nonblack nonraven (an extremely wide category) is not evidence that all nonblack things are nonravens.Nelson Goodman's problem predicate "grue", meaning "observed before 1 January 2050 and blue or observed after 1 January 2050 and green", turns out to be inappropriate because it does not denote a natural kind, according to Quine. He argued that kind-hood was logically primitive: it could not be reduced non-trivially to any other relation among individuals. See also New riddle of induction#Quine.

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