Truth and falsehood exist only in the relation between propositions and actual states of affairs in the universe to which they either correspond (truth) or do not (falsehood). A meaningful proposition says something about the world; a false proposition can still be meaningful, that is, we can understand what it claims: e.g. “apples are furry and bark like dogs.” We know what ‘apples,’ ‘furry,’ and ‘bark’ are, so the statement is meaningful. The falsehood of the proposition lies in the fact that it associates these ideas in a way that does not correspond to the way apples really are in the world.
A ‘truth’ is, therefore, a relation between a proposition and a state of affairs in the world. ‘Truths’ can be known or unknown. We know that the statement “there are trees in the world” is true because there are such trees; that is, we have had experience of them, have sat under them and tasted of their fruit. Thus, this is, for us, at this historical moment and in this cultural context, a known truth.
However, there are other truths that are as of yet unknown because no one has yet discovered states of affairs in the world that correspond to the propositions about whose truth or falsehood we are uncertain. For instance, we don’t know the truth of the matter of the proposition “there is cognitively advanced, conscious life on other planets.” There may very well be or there may not. We do not know, so we cannot yet establish the relation of correspondence between this proposition and the reality of the situation. And this means that we cannot yet know the truth or falsity of this proposition.