People are invariably surprised to hear me say I am both an atheist and an agnostic, as if this somehow weakens my certainty. I usually reply with a question like, “Well, are you a Republican or an American?” The two words serve different concepts and are not mutually exclusive. Agnosticism addresses knowledge; atheism addresses belief. The agnostic says, “I don't have a knowledge that God exists.” The atheist says, “I don't have a belief that God exists.” You can say both things at the same time. Some agnostics are atheistic and some are theistic.