Objection 1. It seems that the accidents of the bread and wine do not remain in this sacrament. For when that which comes first is removed, that which follows is also taken away. But substance is naturally before accident, as is proved in Metaph. vii. Since, then, after consecration, the substance of the bread does not remain in this sacrament, it seems that its accidents cannot remain.
Objection 2. Further, there ought not to be any deception in a sacrament of truth. But we judge of substance by accidents. It seems, then, that human judgment is deceived, if, while the accidents remain, the substance of the bread does not. Consequently this is unbecoming to this sacrament.
Objection 3. Further, although our faith is not subject to reason, still it is not contrary to reason, but above it, as was said in the beginning of this work (I, 1, 6, ad 2; 8). But our reason has its origin in the senses. Therefore our faith ought not to be contrary to the senses, as it is when sense judges that to be bread which faith believes to be the substance of Christ's body. Therefore it is not befitting this sacrament for the accidents of bread to remain subject to the senses, and for the substance of bread not to remain.
Objection 4. Further, what remains after the change has taken place seems to be the subject of change. If therefore the accidents of the bread remain after the change has been effected, it seems that the accidents are the subject of the change. But this is impossible; for "an accident cannot have an accident" (Metaph. iii). Therefore the accidents of the bread and wine ought not to remain in this sacrament.
On the contrary, Augustine says in his book on the Sentences of Prosper (Lanfranc, De Corp. et Sang. Dom. xiii): "Under the species which we behold, of bread and wine, we honor invisible things, i.e. flesh and blood."
I answer that, It is evident to sense that all the accidents of the bread and wine remain after the consecration. And this is reasonably done by Divine providence. First of all, because it is not customary, but horrible, for men to eat human flesh, and to drink blood. And therefore Christ's flesh and blood are set before us to be partaken of under the species of those things which are the more commonly used by men, namely, bread and wine. Secondly, lest this sacrament might be derided by unbelievers, if we were to eat our Lord under His own species. Thirdly, that while we receive our Lord's body and blood invisibly, this may redound to the merit of faith.
Reply to Objection 1. As is said in the book De Causis, an effect depends more on the first cause than on the second. And therefore by God's power, which is the first cause of all things, it is possible for that which follows to remain, while that which is first is taken away.
Reply to Objection 2. There is no deception in this sacrament; for the accidents which are discerned by the senses are truly present. But the intellect, whose proper object is substance as is said in De Anima iii, is preserved by faith from deception. And this serves as answer to the third argument; because faith is not contrary to the senses, but concerns things to which sense does not reach.
Reply to Objection 4. This change has not properly a subject, as was stated above (4, ad 1); nevertheless the accidents which remain have some resemblance of a subject.
Source: Summa Theologica (New Advent)