 An agent who knows what happens after t need not be guided by chances at t. If I’ve seen the coin land heads, that its chance of landing heads was 0.5 at some earlier time is no reason to have my credence in heads be 0.5. Conversely, if all I know is that the chance is 0.5, that’s no reason for my conditional credence in heads to be 0.5 conditional on anything at all. Conditional on it landing heads, my credence in heads is 1, for instance. But given these two restrictions, the Principal Principle seems like a good constraint. Lewis calls evidence about times after t ‘inadmissible’, which lets us give a slightly more concise summary of what the Principal Principle says. For agents with no inadmissible evidence, the rational credence in A, conditional on the chance of A being x, combined with any admissible evidence, is x.