his principle.

So closely like hasty generalization that it cannot be clearly separated
from it is faulty reasoning that arises from neglecting exceptions to a
general principle. All our generalizations, except those that are so
near truisms as to be barren of interest, are more or less rough and
ready, and the process of refining them is a process of finding
exceptions and restating the principle so that it will meet the case of
the exceptions.

Darwin is said to have had "the power of never letting exceptions pass
unnoticed. Every one notices a fact as an exception when it is striking
or frequent, but he had a special instinct for arresting an
exception."[38] It was this instinct which made him so cautious and
therefore so sure in the statement of his hypotheses: after the idea of
natural selection as an explanation of the origin of the species of the
natural world had occurred to him, he spent twenty years collecting
further facts and verifying observations to test the theory before he
gave it to the world. A generalization that the republican form of
government produces greater peace and prosperity than the monarchical
would neglect the obvious exceptions in the Central American republics;
and to make it at all tenable the generalization would have to have some
such proviso as, "among peoples of Germanic race." Even then the
exceptions would be more numerous than the cases which would fall within
the rule.[39] One must cultivate respect for facts in making theories: a
theory should always be held so tentatively that any new or unnoticed
facts can have their due influence in altering it.

Of the errors in reasoning about a cause none is more common than that
known by the older logic as _post hoc, ergo propter hoc_ (after this,
therefore on account of it), or more briefly, the _post hoc_ fallacy.
All of us who have a pet remedy for a cold probably commit this fallacy
two times out of three when we declare that our quinine or rhinitis or
camphor pill has cured us; for as a wise old doctor of two generations
ago declared, and as the new doctrines of medical research are making
clear, in nine cases out of ten nature cures.

Of the same character are the common superstitions of daily life, for
example, that if thirteen sit at table together one will die within the
year, or that crossing a funeral procession brings misfortune. Where
such superstitions are more than playfully held, they are gross cases of
calling that