A Philosophical Essay on ProbabilitiesLaplace, Pierre Simon, marquis de
Philosophy
A Philosophical Essay on Probabilities
Laplace, Pierre Simon, marquis de
Probabilities
3d. The first witness does not speak the truth and the second announces
it. Then a black ball has been drawn from the urn B at the first
drawing, and after having been placed in the urn A a white ball has been
drawn from this urn. The probability of the first of these events is ½
and that of the second is 1000000/1000001; the probability of the
compound event is then 1000000/2000002. Multiplying it by the product of
the probabilities 1/10 and 9/10 that the first witness does not speak
the truth and that the second announces it, one will have
9000000/200000200 for the probability of the event observed relative to
this hypothesis.
4th. Finally, neither of the witnesses speaks the truth. Then a black
ball has been drawn from the urn B at the first draw; then having been
placed in the urn A it has reappeared at the second drawing: the
probability of this compound event is 1/2000002. Multiplying it by the
product of the probabilities 1/10 and 1/10 that each witness does not
speak the truth one will have 1/200000200 for the probability of the
event observed in this hypothesis.
Now in order to obtain the probability of the thing announced by the two
witnesses, namely, that a white ball has been drawn at each draw, it is
necessary to divide the probability corresponding to the first
hypothesis by the sum of the probabilities relative to the four
hypotheses; and then one has for this probability 81/18000082, an
extremely small fraction.
If the two witnesses affirm the first, that a white ball has been drawn
from one of the two urns A and B; the second that a white ball has been
likewise drawn from one of the two urns A´ and B´, quite similar to the
first ones, the probability of the thing announced by the two witnesses
will be the product of the probabilities of their testimonies, or
81/100; it will then be at least a hundred and eighty thousand times
greater than the preceding one. One sees by this how much, in the first
case, the reappearance at the second draw of the white ball drawn at the
first draw, the extraordinary conclusion of the two testimonies
decreases the value of it.
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