A reasonable initial credence function ought to obey the laws of mathematical probability theory; avoid dogmatism, at least by never assigning zero credence to possible propositions and perhaps also by never assigning infinitesimal credence to certain kinds of possible propositions; make it possible to learn from experience by having a built in bias in favor of worlds where the future in some sense resembles the past; and perhaps obey certain carefully restricted principles of indifference, thereby respecting certain symmetries.