He has never thought of the other view; and therefore does not even know that he has never thought of it.

From The Thing: Why I am a Catholic by G.K. Chesterton, published in 1929. The essay is, The Slavery of the Mind.

Chesterton has an interesting perspective and the essay is worth reading but it is cluttered with digressions and illustrations. Stripping the brief argument down to its essence:

I have chosen the subject of the slavery of the mind because I believe many worthy people imagine I am myself a slave. The nature of my supposed slavery I need not name and do not propose specially to discuss. It is shared by every sane man when he looks up a train in Bradshaw. That is, it consists in thinking a certain authority reliable; which is entirely reasonable. Indeed it would be rather difficult to travel in every train to find out where it went. It would be still more difficult to go to the destination in order to discover whether it was safe to begin the journey. . . . What I mean by the slavery of the mind is that state in which men do not know of the alternative. It is something which clogs the imagination, like a drug or a mesmeric sleep, so that a person cannot possibly think of certain things at all. It is not the state in which he says, "I see what you mean; but I cannot think that because I sincerely think this" (which is simply rational): it is one in which he has never thought of the other view; and therefore does not even know that he has never thought of it. . . . The thing I mean is a man's inability to state his opponent's view; and often his inability even to state his own.

Curiously enough, I find this sort of thing rather specially widespread in our age, which claims to possess a popular culture or enlightenment. There is everywhere the habit of assuming certain things, in the sense of not even imagining the opposite things. For instance, as history is taught, nearly everybody assumes that in all important past conflicts, it was the right side that won. Everybody assumes it; and nobody knows that he assumes it. The man has simply never seriously entertained the other notion. . . . Yet nothing can be a more sober or solid fact than that, when the issue was undecided, wise and thoughtful men were to be found on both sides; and the Jacobite theory is not in any way disproved by the fact that Cumberland could outflank the clans at Drummossie. I am not discussing whether it was right as a theory; I am only noting that it is never allowed to occur to anybody as a thought. The things that might have been are not even present to the imagination. But in this age of free-thinkers, men's minds are not really free to think such a thought.

What I complain of is that those who accept the verdict of fate in this way accept it without knowing why. By a quaint paradox, those who thus assume that history always took the right turning are generally the very people who do not believe there was any special providence to guide it. The very rationalists who jeer at the trial by combat, in the old feudal ordeal, do in fact accept a trial by combat as deciding all human history.

[snip]

I could give many other examples of what I mean by this imaginative bondage. It is to be found in the strange superstition of making sacred figures out of certain historical characters; who must not be moved from their stiff symbolic attitudes. Even their bad qualities are sacred. . . . To a simple rationalist like myself, these prejudices are hard to understand.

This deficit is widespread: "The thing I mean is a man's inability to state his opponent's view; and often his inability even to state his own."