Chance as a guide to life: the other principal principle

When choosing between actions having the same two possible outcomes, we should choose the act for which the conditional chance of the preferred outcome, given its performance, is higher. This simple principle is, I argue, both a basic condition of instrumental rationality and the core of our conception of chance. To support this claim, I show that it in the presence of very weak rationality conditions on choice, the principle implies (1) a version of Lewis’s Principal Principle (and that the converse is false), (2) Stochastic Dominance, a principle endorsed by most theories of rational decision making under risk, and (3) that the evidential and causal decision values of an act, conditional on the chances, are the same. Finally, I argue that its appeal as a principle of instrumental rationality derives from the fact that securing a higher chance of a good outcome is better than securing a lower one.