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"Why do humans reason" (PDF), a paper by Hugo Mercier and Dan Sperber, reviewing an impressive amount of research with a lot of overlap with themes previously explored on Less Wrong, suggests that our collective efforts in "refining the art of human rationality" may ultimately be more successful than most individual efforts to become stronger. The paper sort of turns the "fifth virtue" on its head; rather than argue in order to reason (as perhaps we should), in practice, we reason in order to argue, and that should change our views quite a bit. I summarize Mercier and Sperber's "argumentative theory of reasoning" below and point out what I believe its implications are to the mission of a site such as Less Wrong. Human reasoning is one mechanism of inference among others (for instance, the unconscious inference involved in perception). It is distinct in being a) conscious, b) cross-domain, c) used prominently in human communication. Mercier and Sperber make much of this last aspect, taking it as a huge hint to seek an adaptive explanation in the fashion of evolutionary psychology, which may provide better answers than previous attempts at explanations of the evolution of reasoning. The paper defends reasoning as serving argumentation, in line with evolutionary theories of communication and signaling. In rich human communication there is little opportunity for "costly signaling", that is, signals that are taken as honest because too expensive to fake. In other words, it's easy to lie. To defend ourselves against liars, we practice "epistemic vigilance"; we check the communications we receive for attributes such as a trustworthy or authoritative source; we also evaluate the coherence of the content. If the message contains symbols that matches our existing beliefs, and packages its conclusions as an inference from these beliefs, we are more likely to accept it, and thus our interlocutors have an interest in constructing good arguments. Epistemic vigilance and argu