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154 to scholars who have devoted years, let us say, to studying business cycles, or price and wage movements, or the migration and the assimilation of peoples, or the diplomatic purposes of foreign powers. Yet somehow we are all supposed to have opinions on these matters, and it is not surprising that the commonest form of reasoning is the intuitive, post hoc ergo propter hoc.

The more untrained a mind, the more readily it works out a theory that two things which catch its attention at the same time are causally connected. We have already dwelt at some length on the way things reach our attention. We have seen that our access to information is obstructed and uncertain, and that our apprehension is deeply controlled by our stereotypes; that the evidence available to our reason is subject to illusions of defense, prestige, morality, space, time, and sampling. We must note now that with this initial taint, public opinions are still further beset, because in a series of events seen mostly through stereotypes, we readily accept sequence or parallelism as equivalent to cause and effect.

This is most likely to happen when two ideas that come together arouse the same feeling. If they come together they are likely to arouse the same feeling; and even when they do not arrive together a powerful feeling attached to one is likely to suck out of all the corners of memory any idea that feels about the same. Thus everything painful tends to collect into one system of cause and effect, and likewise everything pleasant.