A monthly thread for posting any interesting rationality-related quotes you've seen recently on the Internet, or had stored in your quotesfile for ages.
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Well, 3-SAT is NP-complete, anyway. If consistency checking in mere propositional logic is already NP-complete, then it can't be any easier to do consistency checking to real-world arguments that require predicate logic or other, even more complicated systems to express.
Godel Escher Bach has a section that talks about this.