The fundamental principle of statistical theory that unless there is a reason for believing otherwise, each possible event should be regarded as equally likely.
In this crude form, the principle leads to paradoxes because we can group the alternatives in different ways: the next flower I meet might be blue or red, so its being blue has a probability of one-half; but it also might be blue or crimson or scarlet, so the probability of blue is only one-third).
Evidently we require not mere absence of knowledge of reasons favoring one alternative over another, but knowledge of the absence of such reasons. But this may be hard to achieve, even in apparently symmetrical cases like the outcomes of throwing a die; for example, what do we do about the possibility of its standing on edge, or the fact that the paint on the 'six' side will be heavier than on the 'one' side?
Also see: propensity theory of probability
W C Kneale, Probability and Induction (1949), p.31, 34