This episode introduces "moral uncertainty," the idea that you shouldn't be overly confident in your moral judgments -- like whether it's okay to eat meat, for example, or whether it's okay to abort a baby. The episode's guest is Will MacAskill, a founder of the effective altruism movement and Oxford professor of philosophy. Julia and Will discuss how to take multiple moral systems into account when making a decision, and how to deal with "absolutist" theories that insist some actions have infinite badness, like lying.