For most people, the great mystery of time is that there never seems to be enough of it. If it is any consolation, physicists are having much the same problem. The laws of physics contain a time variable, but it fails to capture key aspects of time as we live it—notably, the distinction between past and future. And as researchers try to formulate more fundamental laws, the little t evaporates altogether. Stymied, many physicists have sought help from an unfamiliar source: philosophers.