According to the late Frederick Copleston one should not dispense of the philosophy of Plato in favor of that of Aristotle, or vice versa. He notes fundamental problems in both systems. Aristotle does not provide a transcendental ground for the constancy of essences. Plato considers universals to exist apart from essences, thus leaving man without direct knowledge of them. Copleston affirms that the Platonic abstraction of the Forms as detached substances needs to be accompanied with the Aristotelian abstraction of the Forms from the immanent subject. This has been done before. He says,
This the Neo-Platonists attempted to do. For example, Plato posited the Forms as Exemplary Causes: the later Platonists placed them in God. With due qualifications, this is the correct view, for the Divine Essence is the ultimate Exemplar of all creatures. (History of Philosophy, Vol 1: Greece and Rome, 297).
This combination of Plato and Aristotle by the Neo-Platonists placed the abstract Form (Plato) into an individual substance (Aristotle). Copleston notes that this combination of these two giants of philosophy was picked up by Thomas Aquinas.
St. Thomas Aquinas, who quotes St. Augustine as to the Divine Ideas, teaches that there is a plurality of ideas in the Divine Mind (S.T., I, 15, 2), rejecting the opinion of Plato that they are “outside” the Divine Mind. He explains that he does not mean that there is a plurality of accidental species in God, but that God, knowing perfectly His Essence, knows imitable by a plurality of creatures. (Ibid.)
In other words, there are not a plurality of Forms in the mind of God, rather there are a plurality of subjects known by him. Aquinas, following after a Neo-Platonic combination of Plato and Aristotle, should not be qualified as a reader of either Aristotle or Plato but both. He does not reject Plato’s understanding of the ideas in toto but modifies it, via Aristotle, for Christian use.