Bayesianism dictates precisely how we revise our beliefs when we learn that some proposition that we had previously entertained is indeed true. But traditional Bayesianism is of little help when it comes to revising one's beliefs in response to entertaining possibilities of which one was previously unaware. According to a Bayesian-inspired principle defended by Karni and Viero (2013, 2015) and endorsed by Bradley (2017), revision in response to growing awareness should not affect the relative probabilities of propositions in one's “old” epistemic state. We argue that this principle is not generally fitting, nor are the arguments that have been offered for the principle compelling. But we formulate and defend a more restricted principle that we take to be the proper extension of Bayesianism to situations of growing awareness.

This talk is based on a joint paper with Katie Steele (ANU).