The principle of
indifference (also called principle of insufficient reason) is a law for
allotting epistemic probabilities. Assume that there are n > 1 mutually
exclusive and jointly exhaustive possibilities. The principle of indifference
states that if the n possibilities are indistinguishable excepting their names,
then each possibility must be assigned a probability equal to 1/n.

In Bayesian
probability, this is the easiest non-informative prior. The principle of
indifference is pointless under the frequency interpretation of probability, in
which probabilities are relative frequencies rather than degrees of belief in
doubtful propositions, conditional upon a state of information.