 Many critics have observed, and Sperber and Wilson have acknowledged, that relevance as defined is not  measurable.[65]  Positive cognitive effects are non-denumerable. Anything a speaker might say or implicate will have infinitely many true contextual implications if it has  any.[66]  We do not know whether processing cost is measured by time, mental effort, electro-chemical energy, inferential steps, or some function of them all. We have no unit of measurement for mental effort, and do not yet know how to measure the energy consumed in processing an utterance. If relevance is not well defined and knowable, it cannot be used to account for specific implicatures.