We do not know what awaits each of us after death, but we know that we will die. Clearly, it must be possible to live ethically-with a genuine concern for the happiness of other sentient beings-without presuming to know things about which we are patently ignorant. Consider it: every person you have ever met, every person you will pass in the street today, is going to die. Living long enough, each will suffer the loss of his friends and family. All are going to lose everything they love in this world. Why would one want to be anything but kind to them in the meantime?
