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.