A Philosophical Essay on ProbabilitiesLaplace, Pierre Simon, marquis de
Philosophy
A Philosophical Essay on Probabilities
Laplace, Pierre Simon, marquis de
Probabilities
There comes up naturally at this point the discussion of a famous
argument of Pascal, that Craig, an English mathematician, has produced
under a geometric form. Witnesses declare that they have it from
Divinity that in conforming to a certain thing one will enjoy not one or
two but an infinity of happy lives. However feeble the probability of
the proofs may be, provided that it be not infinitely small, it is clear
that the advantage of those who conform to the prescribed thing is
infinite since it is the product of this probability and an infinite
good; one ought not to hesitate then to procure for oneself this
advantage.
This argument is based upon the infinite number of happy lives promised
in the name of the Divinity by the witnesses; it is necessary then to
prescribe them, precisely because they exaggerate their promises beyond
all limits, a consequence which is repugnant to good sense. Also
calculus teaches us that this exaggeration itself enfeebles the
probability of their testimony to the point of rendering it infinitely
small or zero. Indeed this case is similar to that of a witness who
should announce the drawing of the highest number from an urn filled
with a great number of numbers, one of which has been drawn and who
would have a great interest in announcing the drawing of this number.
One has already seen how much this interest enfeebles his testimony. In
evaluating only at ½ the probability that if the witness deceives he
will choose the largest number, calculus gives the probability of his
announcement as smaller than a fraction whose numerator is unity and
whose denominator is unity plus the half of the product of the number of
the numbers by the probability of falsehood considered _à priori_ or
independently of the announcement. In order to compare this case to that
of the argument of Pascal it is sufficient to represent by the numbers
in the urn all the possible numbers of happy lives which the number of
these numbers renders infinite; and to observe that if the witnesses
deceive they have the greatest interest, in order to accredit their
falsehood, in promising an eternity of happiness. The expression of the
probability of their testimony becomes then infinitely small.
Multiplying it by the infinite number of happy lives promised, infinity
would disappear from the product which expresses the advantage resultant
from this promise which destroys the argument of Pascal.
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