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.‘
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.
To search within the limits of the familiar and the sensible is to look for your lost keys under the lamp-post, ‘because that’s where the light shines’.
Tautology and Contradiction are the Logico-linguistic limits of the legitimate expression. They mark the boundary of the sensible. Go past that boundary and you are in absurdist territory.
Respectable folks largely live in the zone between Tautology and Contradiction, the mapped terrain of ‘Conventional Understanding’ [vyavharasatya].
Worthwhile Teaching however [and there is not much of it around], begins at this border and moves outward into zones of ever-increasing Absurdity.
The metaphor of the light under the lamp-post goes back to early Sufi literature but has been appropriated as the ‘Streetlight Effect’ by modern Academia. Granting the Sufis credit would have been nice.