1.) Some things we know are abstract
2.) We know things as they are in reality.
3.) No thing in reality is abstract.
If any two are true, the other is false.
I.) Platonism takes 1 and 2 as true and denies 3. Not only is 3 false, it is contrary to the truth: all reality is abstract. Things are particular only as doxa, or in a way we now call “subjective”. We say that something is particular by the same sort of predication as we say that the dental drill is painful. Particularity is as much a feature of how we are relating to an object as pain is a feature of the drill.
It’s not at all clear what “an existent self” would mean on this opinion.
II.) Aristotle takes 1 and 3 as true, but 2 as false. We don’t know things as they are in the same way that we know them as abstract. The way of knowing does not have to be the same as the way it exists, e.g. we know Bemiji on the map as a small dot, but it is not a dot in reality, The letters C, A and T are not a small furry mammal with a tail on one end and a meow on the other.
This is a decent and moderate opinion until one asks how one is supposed to abstract this universal from the particulars without already having it as a criterion for their likeness.
III.) Nominalism takes 2 and 3 as true and 1 as false. This is clear in Berkeley, and Hume both takes his arguments and adds to them. We have no abstract ideas and we can’t imagine what it would be to have one.
When one pushes at this opinion it turns into an appeal for agnosticism: we simply form universals from some power we are completely incapable of describing.