principle of indifference
The principle named (but rejected) by Keynes, whereby if there is no known reason for asserting one rather than another out of several alternatives, then relative to our knowledge they have an equal probability. Without restriction the principle leads to contradiction. For example, if we know nothing about the nationality of a person, we might argue that the probability is equal that she comes from England or France, and equal that she comes from Scotland or France, and equal that she comes from Britain or France. But from the first two assertions the probability that she belongs to Britain must be at least double the probability that she belongs to France. See also Bertrand's paradox.