Does Contemporary Science Refute the Doctrine of Transubstantiation?

Above:  Mosaic of The Last Supper in the Basilica of San Marco in Venice, 13th century. [Information on copyright at]


In 2014, a group of biologists ran a PCR (Polymerase Chain Reaction) DNA amplification on several biological samples to compare their genetic material to known control samples.  There was nothing unusual about the procedure itself; it is used every day in laboratories throughout the world and is a staple of TV crime  investigation dramas.  What was unusual, indeed unprecedented, is that some of the samples tested were consecrated Eucharistic hosts that had been stolen from several Catholic churches in the US and Canada.

These actions were, of course, reprehensible.  Aside from the method of obtaining the hosts, the sacrilegious treatment of what Catholics hold most sacred was a deliberate and gross affront.  The scientists justified their actions as meant to help “misguided” Catholics come to realize the falseness of their religion.

These biologists wanted to show, once and for all, that the Eucharist is not the body of Christ.  If Catholics believe in “transubstantiation,” that the “substance” of the bread becomes the “substance” of the body of Christ — a human body — then there should be some evidence of this change, and the most fundamental place to look for it would be in the DNA of the biological material; or so they argued.

The results of the test showed that all the consecrated hosts gave strong evidence of sections of DNA characteristic of wheat and little to no evidence of sections of DNA characteristic of human beings.  That does it!  End of story… right?  If transubstantiation is true, there should have been no wheat DNA and lots of human DNA; but the actual results were pretty much what you would expect if you grabbed random samples from your local baker.

The thing is, however, that virtually every Catholic priest, bishop, or theologian of the past or present would have predicted exactly the results the scientists got.  None of them would have expected the experiment to show human DNA in the consecrated hosts (except perhaps as the result of contamination) and they would have been astonished if it did.  For Catholic teaching has never held that transubstantiation involves that kind of manifest and measurable change.

Those who ran the test and many who commented on it were quite aware of this fact but insisted nonetheless that any attempts to “get around” the results — for that is how they characterized 2,000 years of philosophical and theological discussion — were just “philosophical weaseling.”

This episode and much of the commentary on it revealed basic misconceptions about the Church’s teachings on the Eucharist.  (Sadly, some polls suggest that many Catholics are similarly confused.)  These misconceptions stem partly from the terminology the Church has traditionally used to express her doctrine in this area.  One misconception has to do with the meanings of these terms as used in theology, which do not perfectly align with their present everyday meanings.  A second misconception has to do with the origin of these terms.  It is widely imagined that they were simply imported into Catholic theology in the Middle Ages from Aristotle’s physics and metaphysics and therefore reflect an archaic understanding of the physical world fundamentally at odds with modern science.

My goal in this article is limited.  It is to address these misconceptions and at least make clear what the Catholic Church does not teach about the Eucharist. That will be enough show that there is no contradiction between the doctrine of transubstantiation and modern science.  This means that I will not be getting into the deep and rich theology of the Eucharist and how it fits together with the rest of Catholic teaching to form a coherent and intelligible whole.  Nor will I be going into the theological reasons for believing in the doctrine, i.e. the abundant evidence from Scripture and ancient tradition that it indeed faithfully expresses a truth revealed by Christ and taught by the Apostles and their successors.

The doctrine of transubstantiation

So, what is the Catholic doctrine of transubstantiation? A good place to turn for an answer is the Catechism of the Catholic Church.  The Catechism has many paragraphs devoted to various aspects of the Eucharist.  When it gets to transubstantiation, in paragraph 1376, it turns to the 16th century Council of Trent, whose statement of the doctrine it quotes in full:

“Because Christ our Redeemer said that it was truly his body that he was offering under the species of bread, it has always been the conviction of the Church of God, and this holy Council now declares again, that by the consecration of the bread and wine there takes place a change of the whole substance of the bread into the substance of the body of Christ our Lord and of the whole substance of the wine into the substance of his blood. This change the holy Catholic Church has fittingly and properly called transubstantiation.”  [Empasis added] (Trent, Session 13, First Decree, Chapter IV)

Throughout the discussions of the Eucharist by the Council of Trent and in the Catechism of the Catholic Church, the words ‘species’ and ‘substance’ appear repeatedly.

What begins as the “substance of bread” with the “species of bread” becomes the “substance of the body of Christ” but with the species of bread remaining. What begins as the “substance of wine” with the “species of wine” becomes the “substance of the blood of Christ” but with the species of wine remaining.

Everything rides on these words ‘species’ and ‘substance’. They are familiar enough in ordinary English, but that familiarity can be misleading.  The word ‘species’ calls to mind biological species.  The word ‘substance’ calls to mind some kind of “stuff” or what things are made of.  However, in much philosophical writing, these terms are used in precise technical senses, especially in the philosophical tradition stemming from Aristotle.  But it is important to note that in theological writing their meanings span a broader range than found in Aristotelian philosophy.  So we should take a closer look at the history of these terms if we are to avoid misunderstandings.

“Species” and “substance”

The word ‘species’ is taken directly from the Latin word specere meaning “to look” (which is where we also get such common words as ‘spectacles’, ‘inspect’, and ‘spectator’).  So originally ‘species’ referred to how something looks, its appearance, shape or form, and consequently it also came to be used to describe the type or category something belongs to.

By the time of the Middle Ages, the term ‘species’ had taken on a very particular role in philosophical discussions of sensation and cognition.  In that context, ‘species’ related, broadly speaking, to the way some physical object is made present in a cognitive faculty, first in the senses, and then in the intellect.  In particular, the expression ’sensible species’ referred to the representation, or cause, or mode in which an external physical object is made present in the senses without actually being there.  The way our eyes can see bread without bread being in our eyes.

St. Thomas Aquinas had his own detailed picture of cognition based on how an external object interacts with some medium (like air or our skin), producing first a species in the medium, then a sensible species in the sense organ, and then an intelligible species in the mind.  While it would be interesting to unpack this Thomistic account, which even today appears both sophisticated and plausible, the point I wish to make here is that many Catholic thinkers in St. Thomas’s own day and since then have disagreed with him, speaking of species and their role in cognition in different ways.  In the three centuries between Aquinas and the Council of Trent, many different Scholastic notions of species arose.  For some thinkers, species were material things that somehow moved between the external object and the observer.  For others, they were some sort of immaterial thing.  For yet others, they were something in between.  And for some, such as William of Ockham, species were not a “thing” at all, but rather all talk about species really referred to a sort of immediate action-at-a-distance by the sense faculty on the external perceived object.1

The crucial point is that when the authors of the Council of Trent used the word ‘species’ they were not committing themselves to any particular physical or metaphysical view of how sensation works.  ‘Species’ simply referred to the overall way that something appears to us, however that works in detail.  More generally, one can say that it referred to the observable features of a thing through whatever modality we observe it.

The second key word, ‘substance’, has a similarly deep philosophical history.  The Latin ‘substantia’ was used to translate the Greek ‘ousia’, an important term in Aristotle’s philosophy.  ‘Ousia’ comes from the Greek verb meaning “to be.”  So the words ‘ousia’ and ‘substantia’ have something to do with what a thing is.  However, the fact that the terms ‘ousia’ and ‘substantia’ were used in the Aristotelian philosophical tradition does not mean that only Aristotelian philosophers used them.  For example, they were terms of central importance in the debates about the Trinity in the early Church, long before —  indeed 900 years before — Aristotelian philosophy came to be positively regarded by most Catholic theologians.2  One need only think of the term “consubstantial” (in the Greek “homoousios”) in the Nicene Creed, which was formulated in 325 AD.

The Latin word ‘substantia’ literally translates as that which “stands under” and came to refer to whatever undergirds all of the various observable aspects and behaviors of a thing and makes it be the thing that it is.  In Aristotelian philosophy it is used in contrast to the “accidents” or “properties” of a thing.

An example may be helpful.  Consider a squirrel.  All the descriptions you might make of the squirrel, all of its observable features — that it is small, bushy-tailed, eats nuts, leaps from branch to branch, is gray (or brown, or black), hibernates in the winter, etc. — refer to “accidents” or “properties” of the squirrel.  What unites all of them together is that they all reside in the same single “substance”, namely a particular squirrel.

As with species, Aquinas had his own understanding of substance, but here too there were a variety of rival views among mediaeval Scholastic philosophers.  For some, the accidents and properties of a thing are knowable by us, but its substance is not and remains forever opaque.  For most others, however, including St. Thomas, the accidents and properties give one access to the substance itself, so that one can learn something about it.  What was common to all the different philosophical views was the idea that the substance of a thing is whatever makes it be the thing it actually is, whether a squirrel, or a tree, or a piece of bread.  So for the authors of the Council of Trent that is all the word ‘substance’ meant, and their use of the term did not imply that they were endorsing or imposing on Catholics any particular detailed philosophical theory, whether Aristotle’s, St. Thomas’s, or anyone else’s.3

Most of those who used the terms ‘species’ and ‘substance’ agreed that they are intimately connected.  It is through the sensible species, the way something appears, that one begins to understand what it is, its substance.  The presence of a certain set of sensible species would strongly suggest the presence of a particular substance.  If something is small, gray, furry, has a bushy tail, eats nuts, and leaps from branch to branch, then it is probably a squirrel.

Both species and substances can undergo change.  Changes in the species of something may or may not indicate a change of the underlying substance.  For example, I haven’t always been 6-foot-3-inches tall, but I have been the same human person, the same substance, for 40 years.  On the other hand, if what was once gray and bushy-tailed now appears flat and red and isn’t moving, it is a good sign that it is no longer a squirrel, at least not a living one.

Going the other way, natural changes of substance always involve changes in the species, because no two substances have exactly the same appearance or observable features.  In transubstantiation, however, the Catholic Church claims that a unique kind of change happens that is never seen in nature: the substance of something changes while the species remain entirely and exactly the same.  That is certainly a very startling and strange claim.  It is also counter-intuitive — by definition, for our intuitions follow our senses, which are shouting at us that in the Eucharist the consecrated bread and wine are still bread and wine.

So, without doubt, what the Church claims happens in the Eucharist is strange, counterintuitive, and has no parallel in nature.  But is it also “unscientific”?  Not in the sense that it contradicts anything science tells us.  For, when the Church says that the “species” of bread remain, it is saying that to all appearances what was bread remains bread. Or, to put it another way, what was bread still behaves like bread under any kind of observation, direct or indirect.  It is not saying that it still looks like bread, but now tastes like something else.  Nor that it still looks and tastes like bread, but now has a different weight.  Nor that it still looks, tastes, and weighs the same as before, but now has a different electrical conductivity.  No: in any way that one could observe it, no change will be found.  In short, the Catholic teaching is that the bread after consecration remains the same in every way on the empirical level.  Consequently, whatever empirical science would predict about it will be found to be true.  And that includes the DNA tests I mentioned at the beginning.

The basic point is that all scientific measurements deal with species; they cannot “measure” substance.  This point is not some bizarre mediaeval notion that belongs in the pre-scientific past.  It is, rather, something widely agreed upon by many modern philosophies of science.  Indeed, it was a core tenet of a movement in the philosophy of science that was dominant in the early decades of the twentieth century, a movement called “positivism.”  The positivists asserted that science deals only with what can be observed and measured, the appearances, which in the terminology of modern philosophy are called the “phenomena” (from the Greek for “that which appears to view”).  According to the positivists, it is unscientific to talk about anything other than the phenomena, including any supposed “underlying reality.”

Positivism came under criticism from various quarters and faded, with more recent philosophers of science generally taking the position that scientific investigation of the natural world, though always starting with what can be observed, the phenomena, can nevertheless learn about the reality that underlies them (which, as we saw, was also the view of St. Thomas and most mediaeval philosophers).4

What all this illustrates is that the distinction between appearances and reality is not just some obscure mediaeval notion, and the question of how to get from experiencing the former to knowing the truth about the latter has been a major preoccupation of modern philosophy from its beginnings with Descartes, Berkeley, Hume and Kant.  These are, indeed, perennial questions even if medieval and modern philosophers approached them from somewhat different angles and used somewhat different terminology to discuss them.

If anything, modern science has brought questions about appearance and reality to the fore and sharpened them considerably.  To take the most important example, in discussions of quantum mechanics there is a hard distinction between what we measure and the mathematical “wave function” that underlies it.  There is debate over the nature of the wave function: is it a straightforward description of what is really there?  Or is it only a tool for calculating what will be measured or observed?  The latter has been the dominant view, but it raises very difficult questions, such as “what is really there when no one is observing?” and “does the mere act of observing change reality?”  In fact, quantum mechanics has its own set of highly counter-intuitive and in some cases shocking claims.  The relation between appearance and underlying reality in quantum mechanics is by no means clear and has been hotly debated for almost a hundred years by both physicists and philosophers.

The foregoing is not to meant to suggest that the paradoxes and puzzles surrounding quantum mechanics have anything directly to do with the puzzles surrounding transubstantiation.  That would be absurd.  Quantum mechanics is a theoretical framework for understanding how things change in the natural world.  Transubstantiation is not a natural change, as already emphasized, but something that has no parallel in nature.  The point is only that quantum mechanics dramatizes how deep and difficult the questions surrounding appearance and reality are, which is why they have been debated since at least the time of Plato.

Why would one believe in transubstantiation?

Another thing that modern science has brought to the fore is the question of evidence, verification, and falsification.  We have already seen that scientific experiments cannot falsify the doctrine of transubstantiation, because the doctrine itself asserts that the bread and wine after consecration remain on the empirical level the same as before.  But, for the same reason, there can be no empirical evidence for transubstantiation, let alone verification.  That would be enough, according to some radically empiricist philosophies (such as positivism), to reject the doctrine as meaningless.  But, whatever one’s philosophy, it raises the question of why a rational person would believe the doctrine.

The answer is actually quite simple: it is believed by Catholics not on empirical grounds, but because God has revealed it.  That does not make the doctrine an easy one to accept.  It is highly counter-intuitive, strange, and different from all other changes that we experience or know about.

But it is not science that made it hard to believe; it has been difficult to accept from the very moment that Christ taught it 2,000 years ago.  The sixth chapter of John’s Gospel recounts the incredulity Christ faced from his followers when he stated the doctrine to them.  He did not respond to that incredulity by weakening his statement of it or by “toning it down.”  Indeed, he seemed to do quite the opposite, by stating it in progressively starker terms.  First, he stated that God would give them bread “which comes down from Heaven and gives life to the world.”  (John 6:33) Then he said that he himself was this “bread of life” come down from Heaven. (John 6:35). Then he said that whoever eats that bread would have eternal life. (John 6:50) And finally he said that the bread they would have to eat to obtain this life was his “very flesh”. (John 6:50, 53) The reaction of his hearers is described in this way: “When many of his disciples heard it, they said, ‘this teaching is difficult, who can accept it?’ … Because of this many of his disciples turned back and no longer went about with him.” (John 6:60,66)


All that I have aimed to show in this article is that the Church’s teaching on the Eucharist in no way contradicts — or could contradict — modern science.  That is an important step, but it does not take one very far.  It does not give any insight into the deep meaning of the Eucharist and what it tells us about the profound realities that lie at the heart of the gospel message: love, sacrifice, and the communion that human beings are meant to have with God and with each other, which will be fulfilled in the “wedding feast” of heaven.5

The doctrine only begins to make sense when we understand that God the Father really did “so love the world that he sent his only Son, that whoever might believe in him might not perish but have eternal life.”  And that this Son humbled himself to take the form of a flesh-and-blood human being.

And that we are meant to have a most profound and intimate union with God through that Son — indeed a “nuptial” union by which we all become “one flesh, one spirit in Christ.”  And that we can therefore be near God in a most profound way whenever we enter a Catholic church and are near the Eucharist, more profoundly than when God was present to the Israelites in the Temple.

And, finally, that we can receive that presence in our very bodies, and through that inherit a share of the life of God himself, a foretaste of the fullness of what we hope to receive in heaven.

That may seem like a lot to swallow for some people. But as Catholics we believe that this amazing gift is exactly what God has promised us. A gift so wonderful that we want nothing more than to share it with all the world.


1.. For an overview of the range of Medieval opinions about sensible species see Robert Pasnau, Metaphysical Themes: 1274-1671 (Oxford, 2011), especially the chapters in Part V on “Quality”.

2.. The word ‘transubstantiation’ was introduced into discussions of the Eucharist in the 11th century, two centuries before the birth of St. Thomas Aquinas and a century before the first translations into Latin of Aristotle’s Physics and Metaphysics.  Aristotelianism, in particular Aristotelian natural philosophy, was still suspect in some theological circles during St. Thomas’s life.

3.. For an overview of the range of Medieval opinions about the notion of substance see Robert Pasnau, Metaphysical Themes: 1274-1671 (Oxford, 2011), especially the chapters in Part II on “Substance”.

4.. The contemporary conversation on “Realism” vs. “Anti-realism” in science (which revolve around the question of whether scientific theories give descriptions of reality or merely help organize observational results) is vast and multifaceted. A recent attempt to propose a middle ground in some of these debates, with a helpful summary and catalogue of the major points at issue is Anjan Chakravartty, A Metaphysics for Scientific Realism (Cambridge, 2009).

5.. For a more detailed account of the Eucharist and the doctrine of transubstantiation from a Thomistic philosophical and theological perspective, see the resources available from the Thomistic Institute, including several short Aquinas 101 videos (,,

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