A Philosophical Essay on ProbabilitiesLaplace, Pierre Simon, marquis de
Philosophy
A Philosophical Essay on Probabilities
Laplace, Pierre Simon, marquis de
Probabilities
The probability that the opinion of each judge is just enters as the
principal element into this calculation. If in a tribunal of a thousand
and one judges, five hundred and one are of one opinion, and five
hundred are of the contrary opinion, it is apparent that the probability
of the opinion of each judge surpasses very little ½; for supposing it
obviously very large a single vote of difference would be an improbable
event. But if the judges are unanimous, this indicates in the proofs
that degree of strength which entails conviction; the probability of the
opinion of each judge is then very near unity or certainty, provided
that the passions or the ordinary prejudices do not affect at the same
time all the judges. Outside of these cases the ratio of the votes for
or against the accused ought alone to determine this probability. I
suppose thus that it can vary from ½ to unity, but that it cannot be
below ½. If that were not the case the decision of the tribunal would be
as insignificant as chance; it has value only in so far as the opinion
of the judge has a greater tendency to truth than to error. It is thus
by the ratio of the numbers of votes favorable, and contrary to the
accused, that I determine the probability of this opinion.
These data suffice to ascertain the general expression of the
probability that the decision of a tribunal judging by a known majority
is just. In the tribunals where of eight judges five votes would be
necessary for the condemnation of an accused, the probability of the
error to be feared in the justice of the decision would surpass ¼. If
the tribunal should be reduced to six members who are able to condemn
only by a plurality of four votes, the probability of the error to be
feared would be below ¼. There would be then for the accused an
advantage in this reduction of the tribunal. In both cases the majority
required is the same and is equal to two. Thus the majority remaining
constant, the probability of error increases with the number of judges;
this is general whatever may be the majority required, provided that it
remains the same. Taking, then, for the rule the arithmetical ratio, the
accused finds himself in a position less and less advantageous in the
measure that the tribunal becomes more numerous. One might believe that
in a tribunal where one might demand a majority of twelve votes,
whatever the number of the judges was, the votes of the minority,
neutralizing an equal number of votes of the majority, the twelve
remaining votes would represent the unanimity of a jury of twelve
members, required in England for the condemnation of an accused; but one
would be greatly mistaken. Common sense shows that there is a difference
between the decision of a tribunal of two hundred and twelve judges, of
which one hundred and twelve condemn the accused, while one hundred
acquit him, and that of a tribunal of twelve judges unanimous for
condemnation. In the first case the hundred votes favorable to the
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