Predictable polarization is everywhere. When we make decisions about what college to attend, or what books to read, or which friends to hang out with, we can usually predict—not with certainty, but with confidence—that doing so will move our opinions in a particular direction. Could this process be (epistemically) rational? Kevin Dorst argues that it can be, if the evidence we get is ambiguous, in the sense that it’s rational to be unsure how to react to it. We explore this argument and an extension of it: by the same logic (says Kevin), it's also rational to interpret evidence in way that favors your prior beliefs. Upshot: polarization and confirmation bias are often rational.