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 wrongmaking property, and then the equiprobability principle in question entails that the a priori probability that a given property \(P\) has the second-order property of being a rightmaking property is equal to the a priori probability that property \(P\) has the second-order property of being a wrongmaking property. Similarly, if one considers instead the family of properties that contains, for example, the second-order property of being a rightmaking property of weight \(W\) and the second-order property of being a wrongmaking property of weight \(W\), the a priori probability that a given property \(P\) has the first of those second-order properties is equal to the a priori probability that property \(P\) has the second of those properties.