transubstantiation in the Summa Theologica  is one of the most famous of his writings,  

in part because his analysis of the mystery  has had a profound effect on subsequent  

official teaching in the Catholic Church.  What does it mean to say that in the Mass,  

the bread and wine consecrated by the priest  truly become the body and blood of Christ?  

Aquinas begins his analysis of the Eucharist  simply by asking whether the bread and wine truly  

become the body and blood of Christ. Should we  believe this? He notes first that Christ himself  

in the Scriptures teaches that the Eucharist  is his true body and blood and that this is the  

universal understanding of the Catholic Church  from the time of antiquity. In the institution  

narrative, Christ says, "This is my body." And  likewise in John 6, "Unless you eat my flesh and  

drink my blood, you have no life in you."

Aquinas notes that Church Fathers like St.  Ambrose of Milan and St. Cyril of Alexandria  

are unequivocal in their assertion of the  Eucharistic conversion of the bread and wine  

into the body and blood of Christ. Furthermore,  the teaching was confirmed in Aquinas's own age  

by the Fourth Lateran Council, which used the  term transubstantiation to describe the mystery.  

So St. Thomas then argues that the Eucharistic  real presence of Christ is not only something  

revealed by God to be believed, but it's also most  fitting. Why? Because it permits the Church to  

enjoy a perpetual presence of Christ down through  history among us that is tangible and concrete  

so that we can dwell with Christ and he with us  for the duration of our pilgrimage in this world,  

both individually and collectively. The  Eucharist also allows us to increase in faith  

meritoriously, by believing in Christ and in his  real presence. Most especially, the Eucharist is  

a tremendous wellspring of grace because it is  the very body and blood of Christ through which  

he gives us a participation in his own charity  and in the sacrificial mystery of the cross.  

Aquinas then asks the question,  

how is Christ present in the Eucharist? What is  the kind of presence in question? In his answer,  

he notes that there are two ways one can become  present in a new way in a particular place.  

One way is by a change of place as when one  travels from London to New York. So one is  

now locally present in New York. The other  is when there is a change of substance, as  

when you build a fire in a given place, so a new  substance or reality is generated like a campfire.  

Or when a child is first conceived and there  is a presence of a new human being in utero.  

The real presence of Christ  

in the Eucharist does not derive from a change of  place as if Christ were to leave the locality of  

heaven and enter into a specific place where Mass  is being said, like Tokyo or Seattle. That is an  

absurd notion. On the contrary, Christ in glory  remains physically unchanged and unaffected in  

his glorified body, even as he becomes present  in many Masses said around the world each day.  

He is perpetually free from being manipulated or  altered by what we do in the Mass. Instead then,  

we must be talking about a change of substance,  as when a fire begins to be present in a place  

it was not previously. Of course, this  is a mysterious supernatural presence,  

not a mere natural reality like building a fire or  conceiving a child. And the reality in question,  

the glorified body of Christ in heaven, already  exists and does not need to come into being.  

But it is rendered present to us in a new location  through the medium of the sacramental elements  

without Christ being altered in himself. 

This brings us to the topic of transubstantiation  as such, which is the total change of the  

substance of bread into the body of Christ and  of the wine into the blood of Christ at the time  

of the words of consecration. Christ comes to be  present like the newly lighted fire you might say.  

He's made present where he previously was not.  

Aquinas brilliantly argues that transubstantiation  is similar to natural generation on the one hand,  

and to creation ex nihilo on the other, and  also dissimilar from both. It is like natural  

generation, where wood changes progressively  into fire and ash, since one thing comes to be  

where there previously was another. But in this  case, the change occurs immediately and at the  

very deepest level of the very being and essence  of things. It's not a mere change of properties  

or of material parts. It is like creation ex  nihilo because God causes something to exist  

in act by his sheer omnipotence as when God  gives existence to the world in creating.  

But it is also somewhat dissimilar, because  

in creation ex nihilo, God causes something new  to be from nothing. While in transubstantiation,  

he causes something new to be from something  else that previously was there. In other words,  

God deploys his almighty power to change one thing  into another. He simply changes bread into the  

body of Christ. How can God do this? Aquinas notes  that God can do it because he has infinite power.  

In fact, it is a merely reasonable thing to admit  that God can do something like this if he wishes  

to. After all, he is omnipotent, so it is feasible  for him and there's no logical contradiction  

entailed by the action. In fact, it would be  unreasonable to say a priori that God simply  

cannot do it or that it is impossible.  

We only know about the mystery of  transubstantiation in faith by grace  

enlightening our judgment, but that does not mean  that the belief is somehow contrary to reason.  

But what about the fact that the host on the  alter looks like bread and has the chemical  

properties of bread and the taste of bread and  so forth? Aquinas notes that there is a real  

distinction in all things between the substance  and the nature of a given thing on the one hand,  

and its properties or accidents, like it's  quantity or qualities on the other.  

For example, there is a distinction  

between a human being as a unified substance,  and his size, weight or hair color, which are  

inherent properties. Normally a substance is  indissociable from its properties, even if some  

of them come and go. Can God however, change the  very substance of bread into the body of Christ  

while leaving the physical properties of quantity  and quality of bread present still in the host?  

He certainly can if he wishes. The action entails  no logical or metaphysical contradiction. There  

are no instances of this kind of action in nature,  but that is part of the point. The Eucharist is  

meant to provide us with a unique form of real  presence. Christ's presence substantially under  

the appearances or accidents of bread and wine.

In this case, why is the presence of  Christ so seemingly hidden or discreet?  

Why leave the accidents of bread and  wine present and appear under them?  

There are several good reasons. First, because the  mystery of the Eucharist is accessible to us only  

by the grace of faith, discovering his presence  in this way has the advantage of nourishing  

supernatural life in us. We are saved by grace,  not by natural acquisitions of knowledge.  

Second,  

this mode of presence respects our capacity  to engage with God freely. The Eucharistic  

presence of Christ does not oblige us to enter  into friendship with God against our will.  

Finally, the accidents of bread and  

wine that remain, symbolize effectively what the  Eucharist is spiritually, food for the journey,  

and an effective source of perpetual spiritual  refreshment. These signs indicate that the  

Eucharist is the sacrament of charity. It sustains  and nourishes the Church in the love of Christ.

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