One could argue that a defining characteristic of human intelligence is our ability to agree on "unreal" concepts. I think the first time I heard it explicitly

One could argue that a defining characteristic of human intelligence is our ability to agree on "unreal" concepts. I think the first time I heard it explicitly was in the book "Sapiens", though I doubt it originated there. In this sense, mathematical concepts are no different than, say Santa Claus. We can agree (mostly) about what it is we're talking about, and talk about it. What sets mathematical "unreal" ideas apart is just how useful they seem to be in describing the universe. They're patterns that fit onto lots and lots of things. For instance, we can all sort of agree on the idea of a (finite) set, and on some basic rules of logic, and with those you can build a lot of mathematics.