Oh, I understand that mathematicians say this, and what their reasoning is (and sat through a Horizon on the subject that had me yelling at the TV) but I still think that 'infinity' is simply a useful concept in mathematics - rather like Shrodinger's Cat is a useful concept in quantum mechanics - but does not actually exist. Just, actually, as any number does not exist - there has to be one of a thing. It is an abstract concept. You cannot have one, or an infinity. You must have one atom or an infinite number of atoms. The first is not abstract - the second, on the other hand, is.