According to reason.
rationality to be closely related, a highly useful remark here is
"A wise man proportions his belief to the
An Enquiry Concerning Human Understanding)
Or to put the same in probabilistic
terms: He is rational who proportions his beliefs according to the
evidence, and he is irrational to the
extent that he does not.
Note that rationality is related to
truth and probability, and is not the same as
reasonability, that is related to
justice. One may be quite rational while being quite unreasonable.
It is more difficult to be reasonable while not being rational, for
fairness depends on true information about who one is fair to. Even
so, there are also reasonable people who are far too
idealistic to be rational.