A System of Logic: Ratiocinative and Inductive, 7th Edition, Vol. II
John Stuart Mill · en
D'Alembert should have put the question in another manner. He should
have supposed that we had ourselves previously tried the dice, and knew
by ample experience that they were fair. Another person then tries them
in our absence, and assures us that he threw sixes ten times in
succession. Is the assertion credible or not? Here the effect to be
accounted for is not the occurrence itself, but the fact of the
witness's asserting it. This may arise either from its having really
happened, or from some other cause. What we have to estimate is the
comparative probability of these two suppositions.
If the witness affirmed that he had thrown any other series of numbers,
supposing him to be a person of veracity, and tolerable accuracy, and to
profess that he took particular notice, we should believe him. But the
ten sixes are exactly as likely to have been really thrown as the other
series. If, therefore, this assertion is less credible than the other,
the reason must be, not that it is less likely than the other to be made
truly, but that it is more likely than the other to be made falsely.
One reason obviously presents itself why what is called a coincidence,
should be oftener asserted falsely than an ordinary combination. It
excites wonder. It gratifies the love of the marvellous. The motives,
therefore, to falsehood, one of the most frequent of which is the desire
to astonish, operate more strongly in favour of this kind of assertion
than of the other kind. Thus far there is evidently more reason for
discrediting an alleged coincidence, than a statement in itself not more
probable, but which if made would not be thought remarkable. There are
cases, however, in which the presumption on this ground would be the
other way. There are some witnesses who, the more extraordinary an
occurrence might appear, would be the more anxious to verify it by the
utmost carefulness of observation before they would venture to believe
it, and still more before they would assert it to others.
§ 6. Independently, however, of any peculiar chances of mendacity
arising from the nature of the assertion, Laplace contends, that merely
on the general ground of the fallibility of testimony, a coincidence is
not credible on the same amount of testimony on which we should be
warranted in believing an ordinary combination of events. In order to do
justice to his argument, it is necessary to illustrate it by the example
chosen by himself.