[drdeca]

I'm not sure that categories don't exist outside of the mind of a person.

I think sets (for example) probably have a real existence, in a sense.

some of the categories we choose might not have anything special about them to mark them as any more real than other categories that we would see as arbitrary, but that wouldn't mean that they aren't real, just that they aren't more real.

But maybe the ontology of abstract objects isn't very relevant here.

Also, I don't mean to say that the categories we use are all arbitrary, just that even the ones which are arbitrary (if any) I think still exist.

Also, I don't right now present any argument for why I believe what I do about abstract objects. I'm just stating beliefs that I have, and which your post seemed(?) to assume something that contradicts.