If one has a family of mutually exclusive properties, and P and Q are any two members of that family, then the a priori probability that something has property P is equal to the a priori probability that it has property Q.

But what underlies this intuitive idea? The answer is a certain very fundamental and very plausible equiprobability principle, to the effect that if one has a family of mutually exclusive properties, and if \(P\) and \(Q\) are any two members of that family, then the a priori probability that something has property \(P\) is equal to the a priori probability that that thing has property \(Q\). For then given that principle, one can consider the family of second order properties that contains the second-order property of being a rightmaking property and the second-order property of being a wrong...