The Immutability Principle

In addition to the principle of by valence, there are a few other principles that I think you've got to invoke some version of in order to get the argument to come out at least plausible. The immutability principle says necessarily propositions don't change their truth values. And if P entails Q, then Q is true and it's not up to the agent at the time whether Q is true. Now we can go ahead and start talking about some principles that are going to be at issue.