Editor’s note: We are delighted to welcome Fr. Michael Chaberek, O.P., as a new contributor. He is the author of Aquinas and Evolution and Catholicism and Evolution: A History from Darwin to Pope Francis.

In my book Aquinas and Evolution I argue that Thomas Aquinas’ teaching cannot be reconciled with theistic evolution. And this holds true regardless of whether Aquinas was right or not regarding the origin of species. My argument is simply that any proposed reconciliation of Aquinas and theistic evolution must do violence to one or the other, or to both. Now, writing for the website Public Discourse, Fr. Nicanor Austriaco, O.P., professor of biology and theology at Providence College, has offered a “Defense of Thomistic Evolution” responding to some of my arguments.

Fr. Austriaco’s responses, however, do not meet the challenges I raised. In fact, in the book, I critique in detail the solutions he proposes. My responses could consist entirely of quotations from Aquinas and Evolution.

Fr. Austriaco begins by offering the example of a mule, which Aquinas treats as a new “kind” that was generated after creation was completed. 

It is clear that Aquinas did not know that organisms evolved. Like most, if not all, persons in Christendom during the thirteenth century, he believed on the authority of divine revelation that most of the organisms belonging to the natural kinds we see in the biological world were created directly by God and reproduced according to their own kind. It is striking, however, that he did acknowledge that at least one biological natural kind, the mule, could not have been directly created by God because it is the offspring of two other natural kinds, an ass and a mare, which God had to create first (cf. Summa theologiae I.73.1 ad 3). Nonetheless, Aquinas acknowledged that the creation of the mule could still be attributed to God because mules “existed previously in their causes.”

In my book, however, I observed that the mule “is not a real species, but merely an infertile combination of two biological species within one family.” (p. 90) Earlier in the book, I argued that for biological macroevolution the relevant understanding of “species” is at the taxonomic level of family. (p. 18-22) The mule does not represent any relevant novelty in terms of biological macroevolution. So the problem for theistic evolution is not where new breeds, variants, races, or even biological species come from, but how new families came about. 

Argument 1. The Lack of Cause

The first argument Fr. Austriaco addresses is on the impossibility of “something lower” generating “something higher,” or the less perfect bringing about the more perfect. There is widespread agreement among both philosophers and scientists that everything must have a proportionate cause. There is no such agreement on whether evolution progresses toward more perfect forms. In fact many biologists (Austriaco quotes Stephen Jay Gould) believe that evolution is just a movement toward greater diversity of life. But if one believes that evolution has gone from bacteria to man, one needs to acknowledge that there has been a perfection of forms across time, at least in a metaphysical sense. Aquinas, obviously, recognizes different levels of perfection among species. Any view, such as Gould’s, that denies “higher” and “lower” in biology contradicts Aquinas.

Fr. Austriaco proposes what he takes to be a counterexample to this perfection principle: the hypothetical evolution of lizards into snakes. Indeed, lizards and snakes are two different natural species. However, there are a few problems with this attempted illustration of evolution.

First, Fr. Austriaco presents the transition from lizard to snake mainly in terms of a loss (not a gain) of biological information. This is a case of devolution, This is not an example of a “less perfect” species giving rise to a “more perfect” one. If a snake is “just a lizard without legs” then it is a good example of the “more perfect” turning into the “less perfect,” not the other way around. 

But there is another, more fundamental problem with the example. To quote Fr. Austriaco, “[F]rom the perspective of evolutionary biology, a snake, for the most part, is a lizard that has lengthened its body, lost its limbs, fused its eyelids, and then made them transparent.” I doubt this is true in biology, but I know it is not true in metaphysics. We could probably list thousands of organic changes that would be necessary to make a lizard look similar to a snake, but we would still not have an actual snake. Indeed, all we get by adding and multiplying changes is a debilitated or dead individual of the same species (natural species). If the changes we apply to a lizard are small enough, the lizard may survive. If they are too big, the lizard will be lame or dead. It will not transform into a snake.

This stems from the second principle of classical metaphysics which states that an accumulation of accidental changes cannot bring about substantial change. In other words, any change we inflict upon a lizard is a change of its individual form, but the changes that occur to an individual will not generate new species since this requires a new substantial form.

It is a principle of metaphysics that substantial forms cannot be created by tinkering with individuals. For this reason, Aristotle believed species were eternal, along with the universe. Aquinas had Biblical revelation at his disposal, therefore he believed that species were created directly by God.

This principle, however, applies only to so-called higher forms, or true substances, not to mere elements or compounds. By tinkering with elements, we may get new compounds or new elements of staggeringly different qualities. This is why the analogy proposed by Fr. Austriaco with mixing hydrogen and oxygen to produce water is misleading. As I wrote in my book:

There are two possible errors in the understanding of this argument. (…) The second error thrives on the misunderstanding of what substance is. Someone could say, if I take salt and water and dissolve salt in water I make salt solution which is a different substance than water or salt. The act of adding salt to water is an accidental change, therefore the accidental change of one substance brings about another substance. Hence, the accidental change results in the substantial change. In this example, however, we do not deal with substances but with merely elements and compounds…Living beings constitute substances in a much stronger sense than non-living beings, to the point that the latter should not even be called substances but elements and compounds. And if we consider a true substance there is no way to transform it into another substance by an accidental change. (p. 49-50)

Argument 2. Disposition of Matter

In responding to my second argument, Fr. Austriaco refers to the “disposition of matter” — the idea that if matter is properly disposed it will generate a new form (species), not directly but indirectly. I included this argument among the arguments proposed by Thomists to reconcile theistic evolution with Aquinas. The bottom line is that matter never generates the form. A full answer can be found in my book, pages 77-78. Instead of extensive quotation, I encourage those interested to read the argument in the book.

Argument 3. No Perfect Nature Is the Cause of Itself

Aquinas’ third argument against theistic evolution is that no perfect being is the cause of its own nature, whereas in biological macroevolution one being would need to cause its own nature, which would then evolve into another nature again caused by an individual.  But, according to Aquinas, no perfect being can be a cause of itself.  (By “perfect” he means higher forms or species, such as reptiles, birds, and mammals, although he explicitly refers to man only.)

In his response, Fr. Austriaco notes that there are two-legged and legless species of lizards, and that perhaps we could produce a two-legged lizard by genetically modifying the four-legged lizard. This is hardly a response to the argument. The experiments with four-legged lizards do not generate anything but lame or dead lizards of the same species. The mere fact that there are other species, such as legless lizards, is no evidence whatsoever of their descent from two or four-legged lizards. If anything, it is another example of biological loss and devolution, which does not explain how macroevolutionary novelties are supposed to emerge. There is no evidence that a four-legged lizard could cause another species, such as a two-legged lizard.

Argument 4. Formal and Efficient Causes Are Missing in Theistic Evolution

In the fourth argument I explained that in theistic evolution, two out of four Aristotelian causes are missing — the formal cause and the efficient cause. Fr. Austriaco writes: “Fr. Chaberek is correct: classical evolutionary theory does not consider formal and final causes.” But I didn’t say this. On the contrary, I said the formal cause is reduced to the final cause. (p. 52) Theistic evolution does entail finality, to the point that it diminishes formality, and this is the actual problem of theistic evolution.

At the same time, we should not confuse one particular theory such as “biological macroevolution” with modern science as such. The problem of finality in theistic evolution is not the same as the problem of finality in modern science. It is possible that science as such is on the right track, but one particular theory may be completely wrong. This is also why challenging biological macroevolution is not equivalent to challenging modern science, or even modern biology alone. I consider the issue of finality in modern science elsewhere in the book, when I discuss the scientific status of the theory of intelligent design. (p. 204-209) It seems, therefore, that the objections to my fourth argument stem from an inattentive reading of my text.

Argument 5. There Is No Established Order of Creation in Theistic Evolution

Aquinas’ fifth argument against theistic evolution stems from the order observed in nature: God wants different degrees of beings to coexist, because these better reveal God’s wisdom and glory. The macroevolutionary interpretation of biology denies this principle, because everything tends to become something else and everything supposedly gains some perfection through the struggle for life and survival of the fittest. This means that there are no degrees of perfection initially established by God. And even if there were, they are not good. Rather, they must be changed, supplemented or perfected by various natural processes. Consequently, theistic evolution denies that creation was ever completed. For Aquinas, however, creation was completed for good with the creation of man.

In his response, Fr. Austriaco quotes Thomas’ teaching on secondary causation. Generally, it is better if creatures participate in the actions of the first cause (God), because this reveals divine wisdom and power more fully. But this is not an answer to the problem raised in my fifth argument. In fact, I included the argument from the “better picture of God” among the eighteen arguments proposed by theistic evolutionists. (argument 3, p. 38) As an answer I should quote the book, but instead of lengthy quotations I encourage readers to consult the book. (p. 68-70)

Fr. Austriaco invokes Thomas’ belief that the multitude and diversity of creatures better reflects divine glory than just a few of them would. (S.Th. I,47,1) “Therefore,” writes Fr. Austriaco, “it is also fitting that God worked via evolution rather than via special creation, because in doing so He was able to produce more species to reflect His glory. Four billion species created over a three-billion-year period is far more than the eight million extant species today.”

Aquinas did not know how many species there were in his time or how many (if any) had gone extinct. But he knew that God could have created as many species as he wanted, whether subsequently over time (as most saints believed) or at one moment (as Augustine believed). In any event, there is no argument for theistic evolution over special creation here because the created species represent divine goodness as much as the supposedly evolved species. If we supplement Aquinas with modern knowledge, we can see God creating billions of species over billions of years.

Fr. Austriaco believes that evolution is a better way of bringing about the diversity of living beings than creation. In his view, “it is also fitting that God worked via evolution rather than via special creation, because in doing so He was able to produce more species to reflect His glory.” But Aquinas claims the opposite: “It is an act of much greater power to make a thing from nothing, than from its contrary.” (S.Th. I,45,5 ad 2) “Creation is more perfect and excellent than generation and alteration.” (S.Th. I,45,1 ad 2)  “Although to create a finite effect does not show an infinite power, yet to create it from nothing does show an infinite power.” (S.Th. I,45,5 ad 3) “It is a greater act to make something according to its entire substance, than to make something according to its substantial or accidental form.” (S.Th. I,45,3, sc)

Aquinas recognizes the value of secondary causes, but secondary causes presuppose the work of creation. Creation itself is a greater act of divine power than the actions of secondary causes. At the same time, the creation of species does not deny the possibility of species being secondary causes in the works of nature. Therefore, there is no reason to propose evolution as a more “fitting” way of producing the universe. In fact, evolution would save the secondary causation but diminish the direct causation, whereas special creation saves both. And this is why creation is more fitting, in addition to being the only realistic explanation of how species could have started to exist.

Photo: Summa Theologica of Thomas Aquinas, by Jonund [CC BY-SA 4.0], from Wikimedia Commons.