From what I understand, a proposition is either true or false, but not both. "This sentence is false" can be neither true nor false and is thus not a proposition.
However, is "This sentence is true" true or false (or both)? And hence, is "This sentence is true" a proposition?
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It's an "anti-paradox", the true or false outcome is totally the choice of the reader, not the statement itself, unlike "This sentence is false", which is self contradicting, and independent of the reader. The reader could themselves can be considered self contradicting, but not the statement, and the statement is self consistent on either choice.
This could explain the apparent contradiction which physicist's have in the "Double slit experiment",where there is an particle/wave duality observed. The observer has the choice of whether a photon, electron, etc, is observed as a particle or wave, by the choice of detection device. The outcome of the experiment is dependent on the observers choice.
The result is explainable if our apparent reality is not material but just pure information ("energy", movement) on a true material reality (the aether). The phenomena of time would be just the sequence of the information.
In electronic circuits, "This statement is false" equivalent is an inverter with feedback (output goes back into the input). If the input is "0" the output is "1", and visa versa. The circuit would become self contradictory, the output oscillating at "0" and "1".
"This statement is true", electronic equivalent is a flip-flop (bistable, one bit memory) with feedback. If the input is "0" the output is "0", and the same with "1". The circuit is self affirming, a stable output, no oscillating.
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"This sentence is true" - call this sentence TT - is not paradoxical. It can be assigned either of the truthvalues T or F. The negation of TT is not the paradoxical Liar sentence "This sentence is false", since the 'this' in these two sentences refer to different sentences. The negation of TT might be rendered as "The sentence 'This sentence is true' is false" - call this sentence NT - which is awkward but quite self-consistent, since TT can indeed consistently be false; in fact, NT can also be true or false: as one would expect, its truthvalue must be the opposite of the truthvalue of TT. It might be worth emphasizing that being a sentence which can be true or false is nothing particularly remarkable. Most sentences of logic are in this category. None of this has anything at all to do with the double-slit experiment, Quantum theory or the nature of physical reality.
The dichotomy sentence/proposition is quite complex to manage, due to its philosophical implications.
See e.g. Nik Weaver,Truth and Assertibility, World Scientific PC (2015), page 4:
Thus, if we want to stay in the realm of propositional logic, we can say that the basic entities are sentences, i.e. linguistic entities, that have a definite truth value.
If so, a sentence like: "This sentence is false", that can be neither true nor false, is not a meaningful sentence to be used in the context of propositional logic.
What about:
Is it paradoxical ? I think so.
Assume that the sentence is true; then its negation: "This sentence is not true" must be false.
But the negated sentence is equivalent to "This sentence is false".
But if "This sentence is false" is false, then the sentence (asserting something about a sentence, i.e. a linguistic entity) "agrees" with the way the things are, and this means that it is true.
Again, we have reached a contradiction.