This is a self confirming sentence, the opposite in some sense to a self contradicting sentence, such as This sentence is false. All other sentences, such as That sentence is false. come somewhere in the middle.

This sentence is true.

Actually, there is more to that statement than appears at first glance. Usually, the above sentence is assumed to be true, which is a natural approach. If we assume that the sentence is true, then the statement that it is true is in fact true, which means that the sentence is true, as we first assumed.

On the other hand, let us now assume the sentence to be false. If the sentence is false, its statement that it is true is actually false. This means that the sentence is in fact false, which agrees with our original assumption.

So we have reached the extraordinary result that the statement is always consistent, regardless of whether or not it is true! In fact, when isolated, the sentence cannot be determined to be either true or false, much like its counterpart, 'This sentence is false, though this one is always consistent within itself, and could be resolved by some external means.'

But this is not an ordinary tautology, such as "all three-legged black cats are black and have three legs". The truth conditions for this sentence are very odd, and it in fact fails to be meaningful for the same reason that "This sentence is false" does, namely that it is self-referential, and the reference cannot be resolved into conditions that can be checked.

Something like "the sentence on this card has nine words" can be self-referential but checkable. It is a physical thing. Truth however is not a property of the physical inscription or utterance, but of the logical statement, and it is that that is unresolvable: what statement? The statement that the statement that... ad infinitum.

The technique you begin with in both cases is to say: "Let's assume it's true. What happens? Now let's assume it's false. What happens?" With a meaningful sentence you might expect one assumption to be consistent and the other to lead to a contradiction, so you could confirm that the consistent assumption is the correct one.

In the case of "This sentence is false", however, both assumptions lead to a contradiction. As Tosta Dojen has pointed out, in the case of "This sentence is true", both assumptions confirm the assumption. This does not prove it to be true. In this case consistency is not sufficient for truth.

With both sentences, "assume it's true" and "assume it's false" fail to resolve its truth. So the remaining alternative is that each of these sentences is neither true not false. They look, especially this one, grammatically meaningful, but you have established logically that they can't have any real meaning. The sense seems to be there, but it cannot be given a reference.

Fuzzled thanks to funnytoes for pointing out that my original addition was not different enough from Tosta Dojen's.

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