Objection 1. It would seem that to create is not to make anything from nothing. For Augustine says (Contra Adv. Leg. et Proph. i): "To make concerns what did not exist at all; but to create is to make something by bringing forth something from what was already."
Objection 2. Further, the nobility of action and of motion is considered from their terms. Action is therefore nobler from good to good, and from being to being, than from nothing to something. But creation appears to be the most noble action, and first among all actions. Therefore it is not from nothing to something, but rather from being to being.
Objection 3. Further, the preposition "from" [ex] imports relation of some cause, and especially of the material cause; as when we say that a statue is made from brass. But "nothing" cannot be the matter of being, nor in any way its cause. Therefore to create is not to make something from nothing.
On the contrary, On the text of Genesis 1, "In the beginning God created," etc., the gloss has, "To create is to make something from nothing."
I answer that, As said above (Question 44, Article 2), we must consider not only the emanation of a particular being from a particular agent, but also the emanation of all being from the universal cause, which is God; and this emanation we designate by the name of creation. Now what proceeds by particular emanation, is not presupposed to that emanation; as when a man is generated, he was not before, but man is made from "not-man," and white from "not-white." Hence if the emanation of the whole universal being from the first principle be considered, it is impossible that any being should be presupposed before this emanation. For nothing is the same as no being. Therefore as the generation of a man is from the "not-being" which is "not-man," so creation, which is the emanation of all being, is from the "not-being" which is "nothing."
Reply to Objection 1. Augustine uses the word creation in an equivocal sense, according as to be created signifies improvement in things; as when we say that a bishop is created. We do not, however, speak of creation in that way here, but as it is described above.
Reply to Objection 2. Changes receive species and dignity, not from the term "wherefrom," but from the term "whereto." Therefore a change is more perfect and excellent when the term "whereto" of the change is more noble and excellent, although the term "wherefrom," corresponding to the term "whereto," may be more imperfect: thus generation is simply nobler and more excellent than alteration, because the substantial form is nobler than the accidental form; and yet the privation of the substantial form, which is the term "wherefrom" in generation, is more imperfect than the contrary, which is the term "wherefrom" in alteration. Similarly creation is more perfect and excellent than generation and alteration, because the term "whereto" is the whole substance of the thing; whereas what is understood as the term "wherefrom" is simply not-being.
Reply to Objection 3. When anything is said to be made from nothing, this preposition "from" [ex] does not signify the material cause, but only order; as when we say, "from morning comes midday"--i.e. after morning is midday. But we must understand that this preposition "from" [ex] can comprise the negation implied when I say the word "nothing," or can be included in it. If taken in the first sense, then we affirm the order by stating the relation between what is now and its previous non-existence. But if the negation includes the preposition, then the order is denied, and the sense is, "It is made from nothing--i.e. it is not made from anything"--as if we were to say, "He speaks of nothing," because he does not speak of anything. And this is verified in both ways, when it is said, that anything is made from nothing. But in the first way this preposition "from" [ex] implies order, as has been said in this reply. In the second sense, it imports the material cause, which is denied.
Source: Summa Theologica (New Advent)