 Essentialists invariably distinguish between what Chomsky (1965) called competence and performance. Competence is what knowing a language confers: a tacit grasp of the structural properties of all the sentences of a language. Performance involves actual real-time use, and may diverge radically from the underlying competence, for at least two reasons: (a) an attempt to produce an utterance may be perturbed by non-linguistic factors like being distracted or interrupted, changing plans or losing attention, being drunk or having a brain injury; or (b) certain capacity limits of the mechanisms of perception or production may be overstepped.