It's been a while since I've thought about epistemology...

Suppose I'm omniscient. This means that I know every true proposition, and also have no false beliefs. Therefore, I must know I'm omniscient, since I'm omniscient is a true proposition. If I know X, then I have X's content as a justified true belief. Therefore, I must be justified in believing that I'm omniscient. What could justify a belief like this? We might be tempted to say that it's justified in virtue of the fact that I'm omniscient. If I knew I was omniscient and discovered a belief in myself, I'd be justified in believing it to be true. But I can't assume my omniscience in justifying that belief -- that's a tight circle. We could do it if we were externalists about justification pretty easily... taking my beliefs as true would be a reliable mechanism. But it'd be kind of surprising if the possibility of an omniscient being implied externalism about justification.