If you are rational, then your attitudes will evolve in a certain way over time. Your beliefs, preferences, and other attitudes will not fluctuate wildly and arbitrarily. Why not? One answer is that there are diachronic requirements of rationality that directly specify how your later attitudes should be related to your earlier ones. A different answer appeals to strict synchronic requirements that uniquely specify what attitudes you ought to have at a time, given your situation at that time. If at all times you obey these strict synchronic requirements, then your attitudes will evolve smoothly over time as a byproduct. I advocate the latter picture, which is a key part of defending a time-slice-centric conception of rationality.