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.
Dan Barker, Godless: How an Evangelical Preacher Became One of America's Leading Atheists
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