You are, by occupation, a campaign manager, and you’ve just been hired by Mortimer Q. Snodgrass, the Green candidate for Mayor of Hadleyburg. As a campaign manager reading a book on rationality, one question lies foremost on your mind: “How can I construct an impeccable rational argument that Mortimer Q. Snodgrass is the best candidate for Mayor of Hadleyburg?”

Sorry. It can’t be done.

“What?” you cry. “But what if I use only valid support to construct my structure of reason? What if every fact I cite is true to the best of my knowledge, and relevant evidence under Bayes’s Rule?”¹

Sorry. It still can’t be done. You defeated yourself the instant you specified your argument’s conclusion in advance.

This year, the Hadleyburg Trumpet sent out a 16-item questionnaire to all mayoral candidates, with questions like “Can you paint with all the colors of the wind?” and “Did you inhale?” Alas, the Trumpet’s offices are destroyed by a meteorite before publication. It’s a pity, since your own candidate, Mortimer Q. Snodgrass, compares well to his opponents on 15 out of 16 questions. The only sticking point was Question 11, “Are you now, or have you ever been, a supervillain?”

So you are tempted to publish the questionnaire as part of your own campaign literature . . . with the 11th question omitted, of course.

Which crosses the line between rationality and rationalization. It is no longer possible for the voters to condition on the facts alone; they must condition on the additional fact of their presentation, and infer the existence of hidden evidence.

Indeed, you crossed the line at the point where you considered whether the questionnaire was favorable or unfavorable to your candidate, before deciding whether to publish it. “What!” you cry. “A campaign should publish facts unfavorable to their candidate?” But put yourself in the shoes of a voter, still trying to select a candidate—why would you censor useful information? You wouldn’t, if you were genuinely curious. If you were flowing forward from the evidence to an unknown choice of candidate, rather than flowing backward from a fixed candidate to determine the arguments.

A “logical” argument is one that fol...