Objection 1. It seems that in the aforesaid words of the forms there is no created power which causes the consecration. Because Damascene says (De Fide Orth. iv): "The change of the bread into Christ's body is caused solely by the power of the Holy Ghost." But the power of the Holy Ghost is uncreated. Therefore this sacrament is not caused by any created power of those words.
Objection 2. Further, miraculous works are wrought not by any created power, but solely by Divine power, as was stated in I, 110, 4. But the change of the bread and wine into Christ's body and blood is a work not less miraculous than the creation of things, or than the formation of Christ's body in the womb of a virgin: which things could not be done by any created power. Therefore, neither is this sacrament consecrated by any created power of the aforesaid words.
Objection 3. Further, the aforesaid words are not simple, but composed of many; nor are they uttered simultaneously, but successively. But, as stated above (Question 75, Article 7), this change is wrought instantaneously. hence it must be done by a simple power. Therefore it is not effected by the power of those words.
On the contrary, Ambrose says (De Sacram. iv): "If there be such might in the word of the Lord Jesus that things non-existent came into being, how much more efficacious is it to make things existing to continue, and to be changed into something else? And so, what was bread before consecration is now the body of Christ after consecration, because Christ's word changes a creature into something different."
I answer that, Some have maintained that neither in the above words is there any created power for causing the transubstantiation, nor in the other forms of the sacraments, or even in the sacraments themselves, for producing the sacramental effects. This, as was shown above (62, 1), is both contrary to the teachings of the saints, and detracts from the dignity of the sacraments of the New Law. Hence, since this sacrament is of greater worth than the others, as stated above (Question 65, Article 3), the result is that there is in the words of the form of this sacrament a created power which causes the change to be wrought in it: instrumental, however, as in the other sacraments, as stated above (62, 3,4). For since these words are uttered in the person of Christ, it is from His command that they receive their instrumental power from Him, just as His other deeds and sayings derive their salutary power instrumentally, as was observed above (48, 6; 56, 1, ad 3).
Reply to Objection 1. When the bread is said to be changed into Christ's body solely by the power of the Holy Ghost, the instrumental power which lies in the form of this sacrament is not excluded: just as when we say that the smith alone makes a knife we do not deny the power of the hammer.
Reply to Objection 2. No creature can work miracles as the chief agent. yet it can do so instrumentally, just as the touch of Christ's hand healed the leper. And in this fashion Christ's words change the bread into His body. But in Christ's conception, whereby His body was fashioned, it was impossible for anything derived from His body to have the instrumental power of forming that very body. Likewise in creation there was no term wherein the instrumental action of a creature could be received. Consequently there is no comparison.
Reply to Objection 3. The aforesaid words, which work the consecration, operate sacramentally. Consequently, the converting power latent under the forms of these sacraments follows the meaning, which is terminated in the uttering of the last word. And therefore the aforesaid words have this power in the last instant of their being uttered, taken in conjunction with those uttered before. And this power is simple by reason of the thing signified, although there be composition in the words uttered outwardly.
Source: Summa Theologica (New Advent)