[Houshalter]

Arguing about the definition of a word is a huge red flag that a discussion is going nowhere. Sadly a lot of philosophy seems to be basically arguing about words.

>Do black holes exist? Electrons? Magnetic fields?

Yes. I mean we might not know what they are exactly or how they work, but they are clearly real phenomena that we can observe. In the same sense, "twoness" is a real property that a set of objects can have.