But he clearly does understand, and some gentle mockery at the other side is called for when they play dumb. I'm sure the fundamental theorem of arithmetic has

But he clearly does understand, and some gentle mockery at the other side is called for when they play dumb. I'm sure the fundamental theorem of arithmetic has a non-obvious proof. And perhaps that's precisely what a mathematician means every time they say "non-obvious". If that were all just made explicit to this general interest site in the first place, perhaps we'd have nothing to discuss. But if we are trying to play coy here, 1+1=2 also requires a non-obvious proof to anyone not versed in formal methods. I looked a proof up: http://mathforum.org/library/drmath/view/51551.html http://mathforum.org/library/drmath/view/51551.html I don't know how long it would take me, working alone, to come up with that proof. It's non-obvious because it took humans probably 100,000 years to come up with it, even though we've had the IQ to do it for a long time. I don't think we could agree on what constitutes a proof of it without some social aspect and convention, so non-obvious by means of proof. But, we've been using and making predictions about the world using 1+1=2 for very much longer. That seems like a pretty worthwhile definition of obvious.