Here is Ludwig Wittgenstein from his Tractatus Logico Philosophicus, a seminal text on the Philosophy of Logic:
‘The Tautology is unconditionally true; the Contradiction is in no condition true…the Truth of Tautology is certain, of Proposition possible, of Contradiction impossible.
Tautology and Contradiction are without sense..Tautology leaves to Reality the whole infinite logical space; Contradiction fills the whole logical space and leaves no point to Reality.
Neither one of them therefore can in any way determine Reality..(They) are the limiting case of the combination of symbols, namely their dissolution.‘
[The latter Wittgenstein, older, wiser, and burdened with a remarkable intellectual integrity, walked away from his youthful fire; but that is another long story.]
O.K. So what in heaven’s name is a ‘Tautology’? I’m glad you asked. For strictly speaking, we don’t know.
What is a Contradiction? We are not too sure either.
But here are examples of what we think they mean:
‘It is raining’ is a proposition. You can verify its truth by looking out the window. ‘It is raining or not raining’ is a Tautology: it’s truth, a Logician would say, is certain. ‘It is both raining and not raining’ is a Contradiction: it’s truth, a Logician would say, is impossible.
‘It is neither raining nor not-raining’ however is Sweet Nonsense. The Logician does not see the need to dignify it with a comment [we’ll meet up with it again on the way to Śūnyam.]