Objection 1. It seems that God has not a speculative knowledge of things. For the knowledge of God is the cause of things, as shown above (Article 8). But speculative knowledge is not the cause of the things known. Therefore the knowledge of God is not speculative.
Objection 2. Further, speculative knowledge comes by abstraction from things; which does not belong to the divine knowledge. Therefore the knowledge of God is not speculative.
On the contrary, Whatever is the more excellent must be attributed to God. But speculative knowledge is more excellent than practical knowledge, as the Philosopher says in the beginning of Metaphysics. Therefore God has a speculative knowledge of things.
I answer that, Some knowledge is speculative only; some is practical only; and some is partly speculative and partly practical. In proof whereof it must be observed that knowledge can be called speculative in three ways: first, on the part of the things known, which are not operable by the knower; such is the knowledge of man about natural or divine thing.
Secondly, as regards the manner of knowing--as, for instance, if a builder consider a house by defining and dividing, and considering what belongs to it in general: for this is to consider operable things in a speculative manner, and not as practically operable; for operable means the application of form to matter, and not the resolution of the composite into its universal formal principles.
Thirdly, as regards the end; "for the practical intellect differs in its end from the speculative," as the Philosopher says (De Anima iii). For the practical intellect is ordered to the end of the operation; whereas the end of the speculative intellect is the consideration of truth. Hence if a builder should consider how a house can be made, not ordering this to the end of operation, but only to know (how to do it), this would be only a speculative considerations as regards the end, although it concerns an operable thing. Therefore knowledge which is speculative by reason of the thing itself known, is merely speculative. But that which is speculative either in its mode or as to its end is partly speculative and partly practical: and when it is ordained to an operative end it is simply practical.
In accordance with this, therefore, it must be said that God has of Himself a speculative knowledge only; for He Himself is not operable. But of all other things He has both speculative and practical knowledge. He has speculative knowledge as regards the mode; for whatever we know speculatively in things by defining and dividing, God knows all this much more perfectly.
Now of things which He can make, but does not make at any time, He has not a practical knowledge, according as knowledge is called practical from the end. But He has a practical knowledge of what He makes in some period of time. And, as regards evil things, although they are not operable by Him, yet they fall under His practical knowledge, like good things, inasmuch as He permits, or impedes, or directs them; as also sicknesses fall under the practical knowledge of the physician, inasmuch as he cures them by his art.
Reply to Objection 1. The knowledge of God is the cause, not indeed of Himself, but of other things. He is actually the cause of some, that is, of things that come to be in some period of time; and He is virtually the cause of others, that is, of things which He can make, and which nevertheless are never made.
Reply to Objection 2. The fact that knowledge is derived from things known does not essentially belong to speculative knowledge, but only accidentally in so far as it is human.
In answer to what is objected on the contrary, we must say that perfect knowledge of operable things is obtainable only if they are known in so far as they are operable. Therefore, since the knowledge of God is in every way perfect, He must know what is operable by Him, formally as such, and not only in so far as they are speculative. Nevertheless this does not impair the nobility of His speculative knowledge, forasmuch as He sees all things other than Himself in Himself, and He knows Himself speculatively; and so in the speculative knowledge of Himself, he possesses both speculative and practical knowledge of all other things.
Source: Summa Theologica (New Advent)