> I don't understand what this is trying to say and this "mathematical model" of sets doesn't seem to be applied further on. I think what is meant by it is tha

> I don't understand what this is trying to say and this "mathematical model" of sets doesn't seem to be applied further on. I think what is meant by it is that we have a set without additional structure. Other structures like vector spaces, groups, fields, etc. are (at least in mainstream maths) also sets, but they're equipped with structure - operations and axioms that govern how these operations work. But if you have just a set, you can't assume anything about the internal structure of what's in that set.