Quine on why modal contexts break substitutivity in equational reasoning
zetalyrae · x · 2026-10-28
zetalyrae shares a classic passage from Quine (with a book page screenshot): modalities and propositional attitudes are notoriously unclear from a logical and philosophical standpoint. Even when Cicero = Tully, substituting one side of a true identity for the other inside a belief or modal context — e.g. "Tom thinks that Tully wrote the Ars Magna" — can flip the truth value, showing how modal contexts break equational reasoning. A timeless puzzle with resonance for how we think about reasoning in LLMs.
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