Wittgenstein introduced the notion of “criteria” in The Blue Book, and it appears frequently in his later writings. The notion almost always appears in the context of the illustrations of certain uses of language. These illustrations typically occur as part of an attempt to clarify or illuminate some aspect of language that has proven philosophically troublesome or contentious, and which has thus served as prod to philosophical theory construction.
Wittgenstein's Blue Book introduction of criteria is embedded in a discussion of our knowledge of states such as pains. A look at this discussion can help us to draw out the nature and purpose of Wittgenstein's use of the notion:
It might be found practical to call a certain state of decay in a tooth, not accompanied by what we commonly call toothache, “unconscious toothache” and to use in such a case the expression that we have toothache, but don't know it. … There is nothing wrong about [this use], as it is just a new terminology and can at any time be retranslated into ordinary language. On the other hand it obviously makes use of the word “to know” in a new way. If you wish to examine how this expression is used it is helpful to ask yourself “what in this case is the process of getting to know like?” “What do we call ‘getting to know’ or, ‘finding out’?”(BB 23; compare PI §§247, 560)
It is clear that we can assign a sense to the expression “unconscious toothache” in the way that Wittgenstein suggests.