So, you want to make the world a better place. You want your net effect on other people’s lives to be positive, for them to be better off because of your actions. This is a noble impulse but one fraught with knotty questions. One of the thorniest: How do you actually know what the consequences of your actions are with any kind of certainty? In particular, how can you understand the consequences of your actions many, many decades into the future? Suppose you’re a firefighter in the 1890s working in the Austrian city of Linz, and you extinguish a flame threatening to consume a whole apartment building. This seems good, until decades later when you realized you prevented Adolf Hitler from dying in childhood. You saved some lives but in doing so failed to save millions more.