The Principal Principle, admissibility, and normal informal standards of what is reasonable

This paper highlights the role of Lewis' Principal Principle and certain auxiliary conditions on admissibility as serving to explicate normal informal standards of what is reasonable. These considerations motivate the presuppositions of the argument that the Principal Principle implies the Principle of Indifference, put forward by Hawthorne et al. (British Journal for the Philosophy of Science, 68, 123-131, 2017). They also suggest a line of response to recent criticisms of that argument, due to Pettigrew (British Journal for the Philosophy of Science, 71, 605-619, 2020) and Titelbaum and Hart (British Journal for the Philosophy of Science, 71(2), 621-632, 2020). The paper also shows that related concerns of Hart and Titelbaum (Thought: A Journal of Philosophy, 4(4), 252-262, 2015) do not undermine the argument of Hawthorne et al. (2017).