Objection 1. It seems that the aforesaid expressions are not true. Because when we say: "This is My body," the word "this" designates a substance. But according to what was said above (1,4, ad 3; 75, 2,7), when the pronoun "this" is spoken, the substance of the bread is still there, because the transubstantiation takes place in the last instant of pronouncing the words. But it is false to say: "Bread is Christ's body." Consequently this expression, "This is My body," is false.
Objection 2. Further, the pronoun "this" appeals to the senses. But the sensible species in this sacrament are neither Christ's body nor even its accidents. Therefore this expression, "This is My body," cannot be true.
Objection 3. Further, as was observed above (4, ad 3), these words, by their signification, effect the change of the bread into the body of Christ. But an effective cause is understood as preceding its effect. Therefore the meaning of these words is understood as preceding the change of the bread into the body of Christ. But previous to the change this expression, "This is My body," is false. Therefore the expression is to be judged as false simply; and the same reason holds good of the other phrase: "This is the chalice of My blood," etc.
On the contrary, These words are pronounced in the person of Christ, Who says of Himself (John 14:6): "I am the truth."
I answer that, There have been many opinions on this point. Some have said that in this expression, "This is My body," the word "this" implies demonstration as conceived, and not as exercised, because the whole phrase is taken materially, since it is uttered by a way of narration: for the priest relates that Christ said: "This is My body."
But such a view cannot hold good, because then these words would not be applied to the corporeal matter present, and consequently the sacrament would not be valid: for Augustine says (Tract. lxxx in Joan.): "The word is added to the element, and this becomes a sacrament." Moreover this solution ignores entirely the difficulty which this question presents: for there is still the objection in regard to the first uttering of these words by Christ; since it is evident that then they were employed, not materially, but significatively. And therefore it must be said that even when spoken by the priest they are taken significatively, and not merely materially. Nor does it matter that the priest pronounces them by way of recital, as though they were spoken by Christ, because owing to Christ's infinite power, just as through contact with His flesh the regenerative power entered not only into the waters which came into contact with Christ, but into all waters throughout the whole world and during all future ages, so likewise from Christ's uttering these words they derived their consecrating power, by whatever priest they be uttered, as if Christ present were saying them.
And therefore others have said that in this phrase the word "this" appeals, not to the senses, but to the intellect; so that the meaning is, "This is My body"--i.e. "The thing signified by 'this' is My body." But neither can this stand, because, since in the sacraments the effect is that which is signified, from such a form it would not result that Christ's body was in very truth in this sacrament, but merely as in a sign, which is heretical, as stated above (Question 85, Article 1).
Consequently, others have said that the word "this" appeals to the senses; not at the precise instant of its being uttered, but merely at the last instant thereof; as when a man says, "Now I am silent," this adverb "now" points to the instant immediately following the speech: because the sense is: "Directly these words are spoken I am silent." But neither can this hold good, because in that case the meaning of the sentence would be: "My body is My body," which the above phrase does not effect, because this was so even before the utterance of the words: hence neither does the aforesaid sentence mean this.
Consequently, then, it remains to be said, as stated above (Article 4), that this sentence possesses the power of effecting the conversion of the bread into the body of Christ. And therefore it is compared to other sentences, which have power only of signifying and not of producing, as the concept of the practical intellect, which is productive of the thing, is compared to the concept of our speculative intellect which is drawn from things. because "words are signs of concepts," as the Philosopher says (Peri Herm. i). And therefore as the concept of the practical intellect does not presuppose the thing understood, but makes it, so the truth of this expression does not presuppose the thing signified, but makes it; for such is the relation of God's word to the things made by the Word. Now this change takes place not successively, but in an instant, as stated above (Question 77, Article 7). Consequently one must understand the aforesaid expression with reference to the last instant of the words being spoken, yet not so that the subject may be understood to have stood for that which is the term of the conversion; viz. that the body of Christ is the body of Christ; nor again that the subject be understood to stand for that which it was before the conversion, namely, the bread. but for that which is commonly related to both, i.e. that which is contained in general under those species. For these words do not make the body of Christ to be the body of Christ, nor do they make the bread to be the body of Christ; but what was contained under those species, and was formerly bread, they make to be the body of Christ. And therefore expressly our Lord did not say: "This bread is My body," which would be the meaning of the second opinion; nor "This My body is My body," which would be the meaning of the third opinion: but in general: "This is My body," assigning no noun on the part of the subject, but only a pronoun, which signifies substance in common, without quality, that is, without a determinate form.
Reply to Objection 1. The term "this" points to a substance, yet without determining its proper nature, as stated above.
Reply to Objection 2. The pronoun "this" does not indicate the accidents, but the substance underlying the accidents, which at first was bread, and is afterwards the body of Christ, which body, although not informed by those accidents, is yet contained under them.
Reply to Objection 3. The meaning of this expression is, in the order of nature, understood before the thing signified, just as a cause is naturally prior to the effect; but not in order of time, because this cause has its effect with it at the same time, and this suffices for the truth of the expression.
Source: Summa Theologica (New Advent)