Well, to be honest, I consider all kinds of numbers equally abstract (with a large "don't know" hanging over what I actually think abstract concepts are), but in terms of something that we know doesn't physically exist imaginary numbers are an easy and obvious choice while you can always handwave about debt and such like for negative numbers. I was tempted, given the context, to reference real numbers with infinite expansions since, in a finite universe, they no more exist that infinity does but I thought that was a rather complex point to make. I just wanted something easy to point at to say "look standard school mathematics! if you want to say infinity doesn't exist, then you have to say this thing doesn't exist either!".