Yet this truism is inordinately difficult to live by, because
every one of us is so little trained in the selection of our samples.
If we believe that a certain thing ought to be true, we can almost
always find either an instance where it is true, or someone who
believes it ought to be true. It is ever so hard when a concrete fact
illustrates a hope to weigh that fact properly. When the first six
people we meet agree with us, it is not easy to remember that they may
all have read the same newspaper at breakfast. And yet we cannot send
out a questionnaire to 816 random samples every time we wish to
estimate a probability. In dealing with any large mass of facts, the
presumption is against our having picked true samples, if we are
acting on a casual impression.
9
And when we try to go one step further in order to seek the causes and
effects of unseen and complicated affairs, haphazard opinion is very
tricky. There are few big issues in public life where cause and effect
are obvious at once. They are not obvious 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.
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