What do you have to be?

[Metonymie]

@ Clive
QED?
What do you mean? I don't get it.
You mean, because an atheist can't be a platonist? I'd agree.

Erm, no. My point was that Joe is an atheist, and believes that triangles exist as immaterial objects, which is similar to Plato's notion of Forms. I don't see a contradiction there.

Ahh
I see what you mean.
But I'd say it depends on how you define "exist".
Plato thought that the material world was fake and that there was a higher level of existence. I don't think that an atheist can believe that and still call himself an atheist with intellectual integrity. He could still not believe in god, of course. But he couldn't come up with a convincing argument for his position.