A System of Logic: Ratiocinative and Inductive, 7th Edition, Vol. II
John Stuart Mill · en
To make this argument valid it must of course be supposed, that the
announcements made by the witness are average specimens of his general
veracity and accuracy; or, at least, that they are neither more nor less
so in the case of the black and white balls, than in the case of the
thousand tickets. This assumption, however, is not warranted. A person
is far less likely to mistake, who has only one form of error to guard
against, than if he had 999 different errors to avoid. For instance, in
the example chosen, a messenger who might make a mistake once in ten
times in reporting the number drawn in a lottery, might not err once in
a thousand times if sent simply to observe whether a ball was black or
white. Laplace's argument therefore is faulty even as applied to his own
case. Still less can that case be received as completely representing
all cases of coincidence. Laplace has so contrived his example, that
though black answers to 999 distinct possibilities, and white only to
one, the witness has nevertheless no bias which can make him prefer
black to white. The witness did not know that there were 999 black balls
in the box and only one white; or if he did, Laplace has taken care to
make all the 999 cases so undistinguishably alike, that there is hardly
a possibility of any cause of falsehood or error operating in favour of
any of them, which would not operate in the same manner if there were
only one. Alter this supposition, and the whole argument falls to the
ground. Let the balls, for instance, be numbered, and let the white ball
be No. 79. Considered in respect of their colour, there are but two
things which the witness can be interested in asserting, or can have
dreamt or hallucinated, or has to choose from if he answers at random,
viz. black and white: but considered in respect of the numbers attached
to them, there are a thousand: and if his interest or error happens to
be connected with the numbers, though the only assertion he makes is
about the colour, the case becomes precisely assimilated to that of the
thousand tickets. Or instead of the balls suppose a lottery, with 1000
tickets and but one prize, and that I hold No. 79, and being interested
only in that, ask the witness not what was the number drawn, but whether
it was 79 or some other. There are now only two cases, as in Laplace's
example; yet he surely would not say that if the witness answered 79,
the assertion would be in an enormous proportion less credible, than if
he made the same answer to the same question asked in the other way. If,
for instance, (to put a case supposed by Laplace himself,) he has staked
a large sum on one of the chances, and thinks that by announcing its
occurrence he shall increase his credit; he is equally likely to have
betted on any one of the 999 numbers which are attached to black balls,
and so far as the chances of mendacity from this cause are concerned,
there will be 999 times as many chances of his announcing black falsely,
as white.