Besides the Real Presence which faith accepts and delights in, there is the doctrine of transubstantiation, from which we may at least get a glimpse of what happens when the priest consecrates bread and wine, so that they become Christ's body and Christ's blood.
At this stage, we must be content with only the simplest statement of the meaning of, and distinction between substance and accidents, without which we should make nothing at all of transubstantiation. We shall concentrate upon bread, reminding ourselves once again that what is said applies in principle to wine as well.
We look at the bread the priest uses in the Sacrament. It is white, round, soft. The whiteness is not the bread, it is simply a quality that the bread has; the same is true of the roundness and the softness. There is something there that has these and other properties, qualities, attributes- the philosophers call all of them accidents. Whiteness and roundness we see; softness brings in the sense of touch. We might smell bread, and the smell of new bread is wonderful, but once again the smell is not the bread, but simply a property. The something which has the whiteness, the softness, the roundness, has the smell; and if we try another sense, the sense of taste, the same something has that special effect upon our palate.
In other words, whatever the senses perceive-even with the aid of those instruments men are forever inventing to increase the reach of the senses- is always of this same sort, a quality, a property, an attribute; no sense perceives the something which has all these qualities, which is the thing itself. This something is what the philosophers call substance; the rest are accidents which it possesses. Our senses perceive accidents; only the mind knows the substance. This is true of bread, it is true of every created thing. Left to itself, the mind assumes that the substance is that which, in all its past experience, has been found to have that particular group of accidents. But in these two instances, the bread and wine of the Eucharist, the mind is not left to itself. By the revelation of Christ it knows that the substance has been changed, in the one case into the substance of his body, in the other into the substance of his blood.
The senses can no more perceive the new substance resulting from the consecration than they could have perceived the substance there before. We cannot repeat too often that senses can perceive only accidents, and consecration changes only the substance. The accidents remain in their totality-for example, that which was wine and is now Christ's blood still has the smell of wine, the intoxicating power of wine. One is occasionally startled to find some scientist claiming to have put all the resources of his laboratory into testing the consecrated bread; he announces triumphantly that there is no change whatever, no difference between this and any other bread. We could have told him that, without the aid of any instrument. For all that instruments can do is to make contact with the accidents, and it is part of the doctrine of transubstantiation that the accidents undergo no change whatever. If our scientist had announced that he had found a change, that would be really startling and upsetting.
The accidents, then, remain; but not, of course, as accidents of Christ's body. It is not his body which has the whiteness and the roundness and the softness. The accidents once held in existence by the substance of bread, and those others once held in existence by the substance of wine, are now held in existence solely by God's will to maintain them.
What of Christ's body, now sacramentally present? We must leave the philosophy of this for a later stage in our study. All we shall say here is that his body is wholly present, though not (so St. Thomas among others tells us) extended in space. One further element in the doctrine of the Real Presence needs to be stated: Christ's body remains in the communicant as long as the accidents remain themselves. Where, in the normal action of our bodily processes, they are so changed as to be no longer accidents of bread or accidents of wine, the Real Presence in us of Christ's own individual body ceases. But we live on in his Mystical Body.
This very sketchy outline of the doctrine of transubstantiation is almost pathetic. But like so much in this book, what is here is only a beginning; you have the rest of life before you.
Taken from Theology for Beginners (c) 1981 by Frank J. Sheed, Chapter 18.