 The Principal Principle says that a rational agent conforms their credences to the chances. More precisely, it says the following is true. Assume we have a number x, proposition A, time t, rational agent whose evidence is entirely about times up to and including t, and a proposition E that (a) is about times up to and including t and (b) entails that the chance of A at t is x. In any such case, the agent’s credence in A given E is x.