A Treatise on Probability

A Treatise on Probability by John Maynard KeynesThe application of probability to conductGiven as our basis what knowledge we actually have, the
probable, I have said, is that which it is rational for us to believe.
This is not a definition. For it is not rational for us to believe that
the probable is true; it is only rational to have a probable belief in
it or to believe it in preference to alternative beliefs. To believe one
thing in preference to another, as distinct from believing the first
true or more probable and the second false or less probable, must
have reference to action and must be a loose way of expressing the
propriety of acting on one hypothesis rather than on another. We
might put it, therefore, that the probable is the hypothesis on which
it is rational for us to act. It is, however, not so simple as this, for
the obvious reason that of two hypotheses it may be rational to act
on the less probable if it leads to the greater good. We cannot say
more at present than that the probability of a hypothesis is one of
the things to be determined and taken account of before acting on it.