In Chapter 62 of Book IV of Summa Contra Gentiles, St. Thomas laid out a number of serious difficulties with crediting the Church’s faith in the Real Presence of Christ in the Blessed Sacrament.
Those difficulties include:
How the Body of Christ comes to be on the altar, how His Body can occupy this precise space that was previously occupied by the bread and wine, and how the Body of Christ can have the appearance, taste, and smell of bread, and His Blood the appearance, taste, and smell of wine, not to mention the capacity of bread and wine to nourish and inebriate, or the capacity to spoil or burn.
St. Thomas provides the idea necessary to resolve these difficulties in Chapter 63. The consecration of the Host and Chalice brings about the transformation of the substance of the bread and wine into the substance of the Body of Christ. The consecration does not entail bringing about the presence of Christ’s Body and Blood in their proper dimensions in space.
This distinction is indeed subtle, but we can understand it, if we think of a loaf of bread.
Let’s say it’s Pepperidge Farm pumpernickel. A distinct substance.
The entirety of Pepperidge Farm pumpernickel-ness is in every slice of the loaf, and in the loaf as a whole, and in even a small bite of one slice. Pepperidge Farm pumpernickel-ness does not, in and of itself, occupy space. It occupies space through the dimensions of the bread, and Pepperidge Farm pumpernickel-ness and the dimensions of the loaf are not the same thing. After all, there are tens of thousands of loaves of the same kind of bread all over the world right now. You don’t have to know how many, in order to know what Pepperidge Farm pumpernickel-ness is.
So if you hold a slice of the bread in your hand to put some mustard on it, you are actually dealing with two things: the reality of Pepperidge Farm pumpernickel-ness, and the size and shape of the slice.
Or think of Coca-Cola. The entirety of Coca-Cola-ness resides in a 12-ounce can, or a 2-liter bottle, or the tank for a soda fountain that dispenses Coke. If you go to Mickey D’s and fill a cup with Coke, you’re actually dealing with two things: Coca-Cola-ness and the size of the cup.
The consecration at Holy Mass involves the transubstantiation of bread and wine into the Body and Blood of Christ, a miracle as great as the creation of the universe out of nothing. It is also involves a second miracle of similar grandeur: the entire dimensions in space of the bread and wine remain, with everything attendant upon that: that is, the sensible qualities and capacities.
With this solution, St. Thomas does not intend to make it easy to believe in the consecration; he cannot do that. The Angelic Doctor concedes, in fact: believing in the Real Presence is super-humanly difficult, and it should be. Only the grace of supernatural faith suffices. We believe by virtue of a gift from heaven.
What St. Thomas does show, however, is: what we believe by supernatural faith about the Blessed Sacrament does not contradict reason; it is not impossible; it is not absurd. When we premise that God Himself brings about the miracle, the whole thing does make sense.
Solution to difficulties regarding the place of the Body and Blood of Christ:
Solution to difficulties regarding the sensible qualities of the consecrated Host and chalice.
Solution to difficulties regarding the capacity of the consecrated sacrament to nourish you or make you drunk, or to rot or burn.