Why this Question Matters.
As we saw in the background document on metaphysics, Plato thought that forms (which he called Ideas) exist in a realm of their own. However, Aristotle considered that forms only exist in so much as they are instantiated in the things they inform. St Augustine, taking a basically Platonic point of view, placed the realm of the Ideas in the mind of God. In this question, Aquinas attempts to reconcile the teaching of St Augustine concerning Ideas in the mind of God with an Aristotelian metaphysical framework.
The Thread of the Argument
A1: Aquinas is quite happy to affirm that there are Ideas in God's mind. In order to do this in an Aristotelian framework, he claims that forms can exist apart from the thing that they inform in two ways: as the exemplar of the thing itself, or as the means of knowing the thing. Aquinas has already covered the idea of a form existing in the mind as a means of knowing something in question 14. But also, since God's knowledge is causative, it is quite natural for Aquinas to accept both ways in which forms can exist in God's mind. Therefore, what Aquinas achieves in this article is to show that Augustine’s placing of the realm of Ideas in the mind of God can be made quite consistent with the former’s Aristotelian approach to God’s causative knowledge.
A2: Having reconciled St Augustine and Aristotle in the first article, Aquinas now faces a tricky problem. God is entirely simple, so it would seem quite natural to assert that there is only one Idea in the mind of God. But this would seem to be quite foreign to St Augustine's conception of Ideas in the mind of God, so it might seem that Aquinas's reconciliation in article 1 is in vain. In his answer, Aquinas is quick to reject an idea (due to Ibn Sina) that would give him an easy way out of this conundrum. He denies that God simply created the first being that consequently created everything else. He asserts that the plans to everything in creation must be in God's mind and therefore there must be a plurality of Ideas in God's mind. Aquinas's answer to the problem of reconciling this with divine simplicity is so disarmingly simple that it may seem like a sleight of hand. He says that “the idea of a work is in the mind of the agent as that which is known, not as the species by which there is knowledge”. In other words, Aquinas is saying that Ideas are present in the mind of God not as a multitude of individual forms facilitating the creation of individual things act by an act, but rather as a single complex facilitating the creation of all things by God's single act of being. The many ideas are in God's mind as objects of his simple knowledge.
A3: Finally, Aquinas asks whether there is an idea in the mind of God corresponding to everything that He knows. The objections list several knowable things that may be considered not to have corresponding forms. Aquinas meets these objections by going back to Plato's fundamental concept that the ideas are sources of the knowledge of things and of their coming into existence. This sits perfectly well with Aquinas's idea of God's creative knowledge and allows Aquinas to claim that ideas and things known to God correspond.