***Cows as we know them have been raised and used by human beings: farmers, butchers,
biologists, cartoonists, and dairy-logo designers - all members of the species Homo sapiens. This may be an obvious fact, but it has important consequence. Cows as we know them are limited and structured by the human brain and human mental capacities. The only cows we know or can know are brain-and-mind-based bovines.***
Do you think talking about bovines is the same as talking about imaginary objects? I can see a bovine. I can touch it. Hear it. Taste it at a good steakhouse, and smell the aroma of a good steak. Sure, maybe it's my imagination that I'm actually seeing, touching, hearing, tasting, and smelling, but you have to make a cut-off somewhere and make certain assumptions which we regard to be true. Those 'assumptions' are best regarded as those things which are necessarily meaningful for our existence (which is why we must accept core assumptions of our physical interaction with the world as being at least somewhat objective statements about the world, eg, there are cows).
I think certain abstract concepts are necessary to believe as 'somewhat objective statements about the world' since some concepts are necessary to impute meaning to our existence, but on the other hand, I think we can do without every mathematical object as actually existing and our world will be no less meaningful.
Warm regards, Harv