It is very hard for a man to defend anything of which he is entirely convinced. It is comparatively easy when he is only partially convinced. He is partially convinced because he has found this or that proof of the thing, and he can expound it. But a man is not really convinced of a philosophic theory when he finds that something proves it. He is only really convinced when he finds that everything proves it. And the more converging reasons he finds pointing to this conviction, the more bewildered he is if asked suddenly to sum them up. Thus, if one asked an ordinary intelligent man, on the spur of the moment, "Why do you prefer civilization to savagery?" he would look wildly round at object after object, and would only be able to answer vaguely, "Why, there is that bookcase . . . and the coals in the coal-scuttle . . . and pianos . . . and policemen." The whole case for civilization is that the case for it is complex. It has done so many things. But that very multiplicity of proof which ought to make reply overwhelming makes reply impossible.
G. K. Chesterton, Orthodoxy, Chapter VI. I was reminded of this salutary passage, expressing a truth too often forgotten about how reasoning really works, by Enbrethiliel's recent post on John Taylor Gatto's Weapons of Mass Instruction.