(science, philosophy of science:) An operative definition is a definition of a concept which explains how it may be observed rather than what it is. Colloquially, an operative definition tells you "how to know it when you see it".

Newton's laws of motion may be seen as an example of an operative definition of force. Note that no physics textbook every defines force; they all seem to rely on this rather axiomatic treatment.

Operative definitions were recognised in physics around the turn of the 20th century. In particular, Ernst Mach was enamoured of the concept; operative definitions remain strong in Einstein's relativity. However, some physical concepts (such as heat) don't seem to have good operative definitions, and today operative definitions are less popular in physics.

In psychology, the idea of operative definition lead to B. F. Skinner's behaviorism. This can be seen as insisting on defining all mental states using only externally visible phenomena (for example, "is smiling" could be part of the definition of "happy"). Such an extreme formulation is metaphysically unpalatable to many people (the author of this writeup among them).

The Turing Test can be seen as an operative definition of intelligence; Searle's Chinese room argument can be understood as a rejection of this operative definition.

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