Although the above discussion has been so far very inconclusive, I find myself believing, at the end of it, that truth and knowledge are different, and that a proposition may be true although no method exists of discovering that it is so. In that case, we may accept the law of excluded middle. We shall define “truth” by reference to “events” (I am speaking of non-logical truth), and “knowledge” by relation to “percepts.” Thus “truth” will be a wider conception than “knowledge.” It would be a practically useless conception, but for the fact that knowledge has very vague boundaries. When we embark upon an investigation, we assume that the propositions concerning which we are inquiring are either true or false; we may find evidence, or we may not. Before the spectroscope, it would have seemed impossible ever to ascertain the chemical constitution of the stars; but it would have been a mistake to maintain that they neither do nor do not contain the elements we know. At present, we do not know whether there is life elsewhere in the universe, but we are right to feel sure that there either is or is not. Thus we need “truth" as well as “knowledge,” because the boundaries of knowledge are uncertain, and because, without the law of excluded middle, we could not ask the questions that give rise to discoveries.
Bertrand Russell (from An Inquiry into Meaning and Truth, 1950)
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