I perhaps need not say much to explain the benefits of logica utens. The value of avoiding inconsistency is not that we thus avoid a specially nasty kind of falsehood, logical falsehood (why should we not follow Emerson, and despise the little minds which fear that hobgoblin?) but that if you are inconsistent you will somewhere be at fault in a nonlogical way: factually wrong if it is an inconsistent story, frustrated if it is an inconsistent plan, morally astray if it is an inconsistent moral code you have. And the value of thinking consequently is that valid reasoning never leads from truth to falsehood; so if all your premises are true, all conclusions validly drawn will be true too; if on the other hand you unexpectedly reach a false conclusion, you have made the possibly unwelcome but salutary discovery that there is error in assumptions you were taking for granted and you must think again. So a man can no more reasonably choose to think inconsequently or inconsistently than to put out his eyes: and pride in being muddleheaded and illogical but getting things done is as fatuous as pride in being an habitually drunken car driver who has never crashed his car—yet.
(P. T. Geach, "On Teaching Logic," Philosophy 54 [January 1979]: 5-17, at 5)
Note from KBJ: Logica utens, according to Geach, is "the practice of thinking logically about this or that subject-matter." It is contrasted with logica docens, which is "the construction of logical theory." He says that the distinction was drawn by "medieval writers."
Note 2 from KBJ: The image to which I linked shows Peter Geach and his wife, G. E. M. Anscombe (1919-2001). This is without question the most high-powered married couple in the history of philosophy. Can you think of another couple that even comes close?
