• Forms simply are essences, not things that have essences. [pace Vlastos around 50ties]

• But the whole theory of self‑predication presupposes that Forms only have essences.

• Aristotle in Metaphysics Zeta.6: Plato thinks that what the thing is is identical to its essence:
  

Again, that to which the essence of good does not belong is not good.‑The good, then, must be one with the essence of good, and the beautiful with the essence of beauty, and so with all things which do not depend on something else but are self‑subsistent and primary. For it is enough if they are this, even if they are not Forms; or rather, perhaps, even if they are Forms. (At the same time it is clear that if there are Ideas such as some people say there are, it will not be substratum that is substance; for these must be substances, but not predicable of a substratum; for if they were they would exist only by being participated in.)

Each thing itself, then, and its essence are one and the same.