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Philosophy

The Logic of Validity and the Semantic Paradoxes

Quick fact

The Liar paradox demonstrates that a single self-referential sentence can be neither true nor false, yet classical logic assumes every declarative sentence is one or the other. This simple example forced logicians to reconsider what 'valid argument' even means.

Why this is interesting

You've heard 'This sentence is false.' If it's true, then it's false; if it's false, then it's true. How can a simple sentence break the very notion of logical validity?