"I never tell the truth, including this sentence."

Then, this sentence is a paradox. Whether you assume it is true or not, you will get contradictory results.

This is the so-called "liar paradox". There are other varieties of it, so I won't list them one by one.

But:

"All my signatures are only 50% credible. Including this sentence. "

This is different from the liar paradox. It can even be considered not a paradox, but an unqualified sentence.

Why do you say that?

First of all, no matter what paradox, it should be expressed in one sentence. Chinese is the same, there can only be one period. You used two periods, but it's actually two sentences. If a sentence can be a proposition, then two sentences may contain two propositions, and there may be various relationships between them, such as contradictions. The two propositions do not necessarily have the same meaning and are harmonious with each other, so even if they are contradictory, it is not surprising.

Secondly, "signature" needs to be defined. You didn't give a definition, and we can't find it in the reference book. You know, unless it is some recognized and indisputable concepts, generally speaking, the introduction of a new concept should be defined. "Signature" is a new concept here, which may lead to misunderstanding if it is not defined.

Third, in general, any kind of signature (whether in the traditional sense or in the sense of the Internet age) does not belong to oral language communication and does not constitute any sentence. Even if written language is regarded as the same as spoken language, it often does not appear in the form of sentences. In other words, your signature may be just a word, a phrase, an emoticon or a picture, but it is not a "sentence" at these times. In addition, sometimes a signature may be two or more sentences. So the "signature" and "sentence" in your signature belong to unclear definition. Unclear and ambiguous sentences are unqualified sentences, so it is not paradox or contradiction.

Fourth, "only 50%" is credible and problematic. It can refer to a de facto ratio or probability. In the former sense, "only 50% credible" means only half credible and half untrustworthy, so in a sentence, half credible and half untrustworthy. In the latter sense, this sentence means that the probability of any signature being credible or untrustworthy is 50%. But as for each signature, it is either credible or untrustworthy, that is, 100%, and there can be no intermediate state, or half of the sentence is credible and half of the sentence is not credible. What the hell do you mean? You can't have two feet at the same time. You just can't express yourself clearly, can you?

Fifth, what do you mean by "credible"? It is also a vague expression. A "trusted signature" may mean that a proposition is true, that is, the assertion I made in my signature conforms to the facts and is credible. It may also be to express "the signature belongs to me", not to be imitated or impersonated by others, which is worth believing.

In short, you may want to create a wonderful and ingenious paradox. But in fact, you just created a signature with vague expression, chaotic logic and little meaning.

I'm sorry, I'm blunt, but I believe you might prefer to hear outspoken criticism. Otherwise, you wouldn't be so interested in abstract disciplines such as logic, philosophy and probability theory.