In his masterpiece Orthodoxy, G.K. Chesterton discussed at some length madmen, and how they are rational but with a very narrow rationality. As he put it:

Every one who has had the misfortune to talk with people in the heart or on the edge of mental disorder, knows that their most sinister quality is a horrible clarity of detail; a connecting of one thing with another in a map more elaborate than a maze. If you argue with a madman, it is extremely probable that you will get the worst of it; for in many ways his mind moves all the quicker for not being delayed by the things that go with good judgment. He is not hampered by a sense of humour or by charity, or by the dumb certainties of experience. He is the more logical for losing certain sane affections. Indeed, the common phrase for insanity is in this respect a misleading one. The madman is not the man who has lost his reason. The madman is the man who has lost everything except his reason.

He goes on to say:

Perhaps the nearest we can get to expressing it is to say this: that his mind moves in a perfect but narrow circle. A small circle is quite as infinite as a large circle; but, though it is quite as infinite, it is not so large. In the same way the insane explanation is quite as complete as the sane one, but it is not so large. A bullet is quite as round as the world, but it is not the world. There is such a thing as a narrow universality; there is such a thing as a small and cramped eternity; you may see it in many modern religions. Now, speaking quite externally and empirically, we may say that the strongest and most unmistakable MARK of madness is this combination between a logical completeness and a spiritual contraction. The lunatic’s theory explains a large number of things, but it does not explain them in a large way.

My experience of antisemitism is exactly this kind of madness. Antisemites are wrong that the Jews control the world, but the chief thing that strikes you, if you ever try to argue with such a lunatic, is how incredibly small the world he lives in is. They often play a kind of “where’s Waldo” game of spotting Jewish people in vicinity of important events, which they’re so caught up in that they never stop to ask what part the Jew they’ve identified played in the event. If they watched the movie Forest Gump (whose conceit was that a man named “Forest Gump” happened to be in the vicinity of every important public event in the lives of the baby boomers) and they applied the same logic, they would conclude it was a movie about the man who ruled the world.

There is a strange intellectual collapse that goes on when a man becomes an antisemite. I suspect that the causality runs from the mental collapse to antisematism; the man grows tired of living in a complex world and so retreats into the fantasy of the world being easily intelligible, and for various historical reasons the Jews make excellent fodder for this kind of fantasy.

I want to be clear, since this is the internet, that I do not explain the thing in order to excuse it. I’m interested in the explanation because understanding what has gone wrong is the first step in trying to help someone, if they can be helped.