A Philosophical Essay on ProbabilitiesLaplace, Pierre Simon, marquis de
Philosophy
A Philosophical Essay on Probabilities
Laplace, Pierre Simon, marquis de
Probabilities
accused warrant in thinking that the proofs are far from attaining the
degree of strength which entails conviction; in the second case, the
unanimity of the judges leads to the belief that they have attained this
degree. But simple common sense does not suffice at all to appreciate
the extreme difference of the probability of error in the two cases. It
is necessary then to recur to calculus, and one finds nearly one fifth
for the probability of error in the first case, and only 1/8192 for this
probability in the second case, a probability which is not one
thousandth of the first. It is a confirmation of the principle that the
arithmetical ratio is unfavorable to the accused when the number of
judges increases. On the contrary, if one takes for a rule the
geometrical ratio, the probability of the error of the decision
diminishes when the number of judges increases. For example, in the
tribunals which can condemn only by a plurality of two thirds of the
votes, the probability of the error to be feared is nearly one fourth if
the number of the judges is six; it is below 1/7 if this number is
increased to twelve. Thus one ought to be governed neither by the
arithmetical ratio nor by the geometrical ratio if one wishes that the
probability of error should never be above nor below a given fraction.
But what fraction ought to be determined upon? It is here that the
arbitrariness begins and the tribunals offer in this regard the greatest
variety. In the special tribunals where five of the eight votes suffice
for the condemnation of the accused, the probability of the error to be
feared in regard to justice of the judgment is 65/256, or more than ¼.
The magnitude of this fraction is dreadful; but that which ought to
reassure us a little is the consideration that most frequently the judge
who acquits an accused does not regard him as innocent; he pronounces
solely that it is not attained by proofs sufficient for condemnation.
One is especially reassured by the pity which nature has placed in the
heart of man and which disposes the mind to see only with reluctance a
culprit in the accused submitted to his judgment. This sentiment, more
active in those who have not the habitude of criminal judgments,
compensates for the inconveniences attached to the inexperience of the
jurors. In a jury of twelve members, if the plurality demanded for the
condemnation is eight of twelve votes, the probability of the error to
be feared 1093/8192, or a little more than one eighth, it is almost 1/22
if this plurality consists of nine votes. In the case of unanimity the
probability of the error to be feared is 1/8192, that is to say, more
than a thousand times less than in our juries. This supposes that the
unanimity results only from proofs favorable or contrary to the accused;
but motives that are entirely strange, ought oftentimes to concur in
producing it, when it is imposed upon the jury as a necessary condition
of its judgment.
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