In his recent book, Aquinas and Evolution (Chartwell Press, 2017), my Dominican brother Fr. Michael Chaberek, O.P. has argued that Aquinas’s thought cannot be reconciled with evolutionary biology without doing irreparable violence to the Thomistic synthesis. He is convinced that the Thomistic philosophical framework is incompatible with Darwinian evolution.

If Aquinas were alive today, how would he think through the process of evolution? Would he conclude that it is philosophically impossible?

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 this essay, I would like to examine the five reasons/objections that Fr. Chaberek presents in the second chapter of his book for why Aquinas’s teaching, in principle, necessarily excludes evolution. I will provide rebuttals to illustrate the coherence of a Thomistic account of evolution. My basic thesis is that on Aquinas’s own terms, it is still philosophically intelligible to claim that God created through evolution.

Objection 1: “According to theistic evolution, the lower (i.e., less perfect) cause can lead to the higher effect (i.e., more perfect). But in Aquinas’s view, no being can convey more act than it possesses.”

RESPONSE: As I read Aquinas here, a more perfect being is something that has more capacities, more powers, than a less perfect substance. These novel capacities and powers constitute it as something that has “more act” than its less perfect counterparts. With this in mind, Fr. Chaberek’s objection is a robust one based on a foundational metaphysical principle in the Aristotelian-Thomistic synthesis. The less perfect cannot make the more perfect. In everyday language, the principle is relatively straightforward: You cannot give what you do not have.

In response, let us consider a specific example from evolutionary biology: the evolution of snakes from lizards. The consensus among evolutionary biologists is that snakes evolved from lizards over a hundred-million-year period. To put it another way, from 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.

Notably, evolutionary developmental biologists have already identified some of the genetic events that could have contributed to this evolutionary transformation. For example, there are data that reveal that mutations in the Sonic Hedgehog (Shh) gene enhancer can explain the loss of the rear limbs in the python. There are other data that reveal that mutations in the Hox genes can explain the transformation of the lizard’s spinal column from lumbar vertebrae without ribs to thoracic vertebrae with ribs. (A snake has an extended thoracic region compared to that of a lizard.) Finally, we have also identified the molecular mechanism to increase the number of vertebrae in an animal spine. (A typical snake has over 300 vertebrae, while a typical lizard only has around sixty-five vertebrae. Humans have thirty-three vertebrae.)

For the sake of brevity, I summarize this evolutionary transformation that gave rise to the first snake using the following equation:

lizard 1 + lizard 2 → snake

Here, we imagine mating pairs of lizards, where each lizard has a unique set of mutations, which, when put together into a single genome through mating, specify snakes with elongated bodies, no limbs, and fused transparent eyelids.

Fr. Chaberek’s objection would apply here: How could two individual lizards, each a substance with lizard nature, interact to give rise to a snake, which is a substance with a snake nature? (Note that the snake nature has novel aspects like transparent fused eyelids that lizard natures lack.) Fr. Chaberek claims that they could not. In his view, therefore, evolutionary theory is ruled out on philosophical grounds.

Before I respond to Fr. Chaberek’s objection to evolution, I think that it is important to point out that his philosophical objection is an objection not only to modern biology but to modern chemistry as well. Consider the following equation from basic high school chemistry:

hydrogen + oxygen → water

Here, the substance of hydrogen interacts with the substance of oxygen to give rise to the substance of water.

Fr. Chaberek’s objection applies here too. Philosophically, how can two unique substances of two different natural kinds, hydrogen and oxygen, give rise to another substance of another natural kind, water, with distinctive properties and chemical traits that they themselves do not have?

Clearly, I think that the claims of modern chemistry are true, and that hydrogen and oxygen can indeed interact to generate water. Fr. Chaberek, himself, I am sure, would agree. How then do we explain this chemical transformation philosophically from within the Thomistic tradition?

We have two options.

First, as Aquinas himself did, we could include the natural causality of other more perfect natural substances that mediate this and all the other chemical transformations that chemists study in our explanation of how chemistry works.

In his own day, Aquinas believed that the causality of the angels and of the celestial spheres or heavenly bodies—perfect substances thought to exist in the heavens by the best thinkers of his day—was involved in all natural processes of change (cf. De potentia q5 a8; De veritate q5 aa 8-9; De operationibus occultis). Since the angels and the celestial spheres are more perfect than any material thing, they would give to hydrogen and to oxygen the perfections that they lack so that together they can generate water.

Second, we could forsake natural causality entirely and include the supernatural causality of God in our philosophical explanation of everyday chemical transformations. Here God, who is the most perfect of all beings, would provide the perfections lacking to hydrogen and to oxygen so that with His help, they could together generate water.

Returning to evolutionary biology, I think that we can explain evolutionary transformations using the same philosophical strategies summarized above to explain chemical ones. We could invoke the causality of more perfect natural substances—in the absence of celestial spheres, we could speak about the causality of angels alone—or we could invoke the causality of God. Aquinas himself appeals to the power of the heavenly bodies to explain how fraternal twins of different sexes are created (cf. De veritate q5 a8 ad9). The esteemed twentieth-century Thomist philosopher, Jacques Maritain, chose the second option when he posited the existence of a “superelevating” motion of the First Cause working in tandem with the immanent activity of organisms to explain biological evolution.

Regardless of how one chooses to philosophically explain evolutionary transformations (and chemical transformations, for that matter!), these conceptual strategies suggest credible approaches to respond to Fr. Chaberek’s first objection to the tenability of a Thomistic account of evolution. If hydrogen and oxygen can together produce water, then two individual lizards should, in principle, be able together to produce a snake.

Objection 2: “The second reason theistic evolution contradicts Aquinas’s doctrine is that it presupposes that the nature (or the substantial form) of a living being can be changed into a different nature by an accidental change. This, however, is impossible in Aquinas’s view: accidental change can lead to only accidental changes, whereas a change of nature requires substantial change.”

RESPONSE: To understand this objection, we first need to review Aquinas’s account of change. Like Aristotle, Aquinas acknowledged the commonsense view that there are two kinds of change in the world. The first type, called substantial change, involves radical change that transforms an individual of one kind into an individual of another kind. A human being becoming a corpse is an example of this kind of change. The second type, called accidental change, involves superficial change that transforms an individual without changing his fundamental identity. A fat human being becoming a thin human being after an all-fruit diet is an example of this second kind of change.

To explain these two kinds of changes, Aquinas, again like Aristotle, posited the existence of two constituents of individual things. First there is form, which is that constituent that explains what something is, and another correlative constituent called prime matter that explains how that something can perdure through change. Every individual thing is made up of prime matter and a substantial form that informs it. For material things, one cannot have matter without form, nor form without matter.

In Aquinas’s view, therefore, substantial changes involve a transformation of substantial form in which prime matter remains the same. Thus, when a human being dies, his prime matter, once informed by his human substantial form, is now informed by the substantial forms of the elements, primarily carbon and hydrogen.

Given Aquinas’s account of change, it is clear that every evolutionary transformation, like the lizard-to-snake transformation discussed above, must necessarily involve a change in substantial form. It requires a substantial change.

Fr. Chaberek objects that theistic evolution assumes “the nature (or the substantial form) of a living being can be changed into a different nature by an accidental change . . . This, however, is impossible in Aquinas’s view: accidental change can lead to only accidental changes.” I agree and disagree.

Fr. Chaberek is correct when he says that accidental changes cannot lead to substantial changes directly. However, I do think that accidental changes can lead to substantial changes indirectly. They do this by changing prime matter’s disposition to substantial form.

Consider the directionality and specificity of change. When a human being dies, he becomes a corpse, and not a kaleidoscope of butterflies. When a log burns in a fire, it becomes ashes and not a puddle of water. To explain this specificity and directionality of change, Aquinas appealed to the disposition of prime matter.

In Aquinas’s view, a substantial form limits the disposition of prime matter such that logs become ashes and not water, because the matter of logs is predisposed, by the substantial form of wood, to the substantial forms of the elements in ashes rather than to the substantial form of water. In the case of human death, a human being becomes a corpse rather than butterflies, because the human substantial form predisposes the matter in the human body to the forms of carbon and hydrogen and not to the substantial forms of butterflies.

Importantly, Aquinas also appeals to the disposition of prime matter to explain how different individuals of the same natural kind can possess natural traits to different degrees. Thus, he will explain that a person with exceptional intelligence is an individual whose matter is better disposed to his human form than is typical of human beings (cf. In II de Anima, 19).

Returning to Fr. Chaberek’s objection, I propose that accidental changes can lead to substantial changes indirectly, because accidental changes can change the predisposition of matter to a substantial form. Aquinas wrote: “Every substantial form requires a proper disposition in its matter, a disposition without which it cannot exist. This is why alteration is the way to generation and to corruption” (De mixtione elementorum).

Recall my example of an accidental change where a fat man becomes a thin man after an all-fruit diet. If we alter this example slightly, we get the following scenario: A fat man becomes a thin man who becomes a dead man after a month-long hunger strike. Biologically, we would say that dehydration and starvation led to multiple organ failure and the death of the hunger striker, but how do we explain this process of change philosophically? We would have to say that the accidental changes that gradually led to the decrease in the man’s overall mass also altered the predisposition of his matter such that a substantial change, i.e., his death, inevitably occurred.

In the same way, I propose that genetic mutations can also affect the predisposition of matter. Recalling Aquinas’s explanation for the superior intelligence of certain persons, for example, I think that an individual with Down syndrome is not as intelligent as the rest of the human population because his extra twenty-first chromosome alters the predisposition of his matter so that it is not as predisposed to his substantial form as it is in his peers.

Therefore, contrary to Fr. Chaberek’s objection, in my view, accidental changes can lead to substantial changes indirectly. They do this by changing matter’s disposition to substantial form.

Objection 3: “The third reason is that theistic evolution presupposes that one nature can be the cause of another nature. In contrast, according to Aquinas no ‘perfect thing’ produces its own nature, but only participates in the nature that it inherits . . . Thus, a man cannot be a cause of mankind, a dog cannot cause dog’s nature, a cat, a cat’s nature, and so on. But if a being is not a cause of its own nature, much less can it produce a different nature (ScG III.69 c).”

RESPONSE: Fr. Chaberek is correct. According to Aquinas, a being cannot cause its own nature, much less produce a different nature. However, this philosophical objection does not rule out evolutionary transformations.

Taking what we have already said to respond to Fr. Chaberek’s first two objections to Thomistic evolution, we can now provide the following philosophical account for evolutionary transformation that does not require that one nature be the cause of another nature.

Genetic mutations lead both to accidental changes and to changes in the disposition of matter. For example, take the following hypothetical scenario: a mutation in the Sonic Hedgehog (Shh) gene enhancer could lead to four-limbed lizards giving rise to lizard offspring that lack rear limbs. This would be accidental change. However, philosophically speaking, this Shh mutation would also alter the disposition of the two-legged lizard’s matter such that it is now predisposed to the novel substantial form that specifies two-legged lizards as a distinct natural kind. If this mutant two-legged lizard were to mate with another mutant lizard bearing the same Shh genetic mutation, then they would generate lizard offspring that permanently lacked rear limbs. This would be substantial change. Strikingly, stable two-legged lizard species exist today that have lost either both of their front limbs or both of their hind limbs. There are even species of legless lizards that look like snakes but differ from snakes in other ways!

Here the generation of the new natural kind of two-legged lizards would not be attributed to the causality of the four-legged lizard—Fr. Chaberek’s concern in his objection—but to the causality of the First Cause working with and through creaturely causality in the process we call evolution.

An objector may point out that most genetic mutations are deleterious. They alter the disposition of matter such that progeny that bear these mutations are sickly and die. This is true. However, not all mutations are necessarily deleterious. As we saw above with the legless lizards, the loss of lizard limbs is not inherently lethal. Though we will never really know the reasons for the survival advantage that legless lizards have over their legged counterparts in a particular environmental scenario, it should not be too hard to see how mutant legless lizards would be able to burrow better into the ground to avoid predation or to chase prey than their four-legged parents.

Objection 4: “The fourth reason is that according to Aquinas (following Aristotle) every composite thing has four causes: final, formal, efficient, and material. In theistic evolution, the efficient cause of species is the power of generation only guided by the final cause, who is God. Thus, even though theistic evolution involves final causality, it reduces ‘down’ efficient causation to accidental changes in matter and the operations of nature (medieval ‘movements of heavens’). And, according to Aquinas, this kind of cause cannot generate new natures (as was shown in the third problem) . . . And since it is formal cause (not matter), which produces the substantial form, theistic evolution lacks the formal cause, which is reduced ‘up’ to the final cause alone. Therefore, theistic evolution lacks both formal cause and efficient cause.”

RESPONSE: Fr. Chaberek is correct. The scientific revolution that replaced Aristotle’s substantial biology with Descartes’s, Boyle’s, and Locke’s mechanical biology in the seventeenth century set out to discard formal and final causality. However, as I have written elsewhere at much greater length, today’s genomic revolution has challenged biologists to recover a holistic account of the organism that embraces all four Aristotelian causes. I will only briefly summarize this view, which I call the systems hylomorphic view, here.

Today, systems biologists view the organism as a dynamic network of molecular interactions over time. Here, the human animal is a substantial being, a dynamic network of macromolecules now existing not as independent molecules per se but as different virtual parts of the one human organism. This species-specific network, which is distributed in three-dimensional space, and which is able to interact over time in the robust, self-organizing process that we call human development, in my view, is a manifestation of the human being’s substantial form. The correlative metaphysical principle that is actualized by that substantial form would be its matter.

Human embryos become human adults and puppies become dogs because of their networks of molecular interactions. Thus, the systems account described above not only emphasizes the holism (and thus the formal cause of the organism) but also the end-directedness of its development (and thus its final cause).

Fr. Chaberek is correct: classical evolutionary theory does not consider formal and final causes. However, he does not seem to realize that this is not a necessary aspect of the theory. There are already accounts of evolution that acknowledge that evolving organisms are end-directed, holistic, and dynamic systems. These dynamic systems accounts reveal that evolutionary theory can properly take efficient, material, formal, and final causality into consideration.

Objection 5: “The fifth reason is that, according to Aquinas, God wanted different degrees of perfection in nature. This happens among different species, as well as within one organism—among its organs . . . Hence according to Aquinas, things less perfect and more perfect exist for the sake of the greatest perfection of the whole material world. This order is intended by God . . . And this is contrary to the evolutionary view of nature, in which each part requires continual change toward greater perfection in ‘struggle for life’ and ‘survival of the fittest.’”

In my view and the view of other contemporary proponents of theistic evolution, God created through evolution. Why did God create this way? For at least two reasons.

First, for the Catholic tradition, the answer to the purpose-of-creation question is clear: God chose to create because He wanted to manifest and communicate His glory. The Catechism of the Catholic Church, the authoritative summary of Catholic doctrine, proclaims that “Scripture and Tradition never cease to teach and celebrate this fundamental truth: ‘The world was made for the glory of God.’”

How does God communicate His glory to His creatures? According to Aquinas, God communicates His glory to His creatures by giving them a participation in His existence. However, he also explains that God shares His perfections with His creatures by inviting them to participate in His causality, which manifests itself in His governance of His creation:

But since things which are governed should be brought to perfection by government, this government will be so much the better to the degree that the things governed are brought to perfection. Now, it is a greater perfection for a thing to be good in itself and also the cause of goodness in others, than only to be good in itself. Therefore God so governs things, that He makes some of them to be causes of others in government, like a master, who not only imparts knowledge to his pupils, but gives also the faculty of teaching others. (Summa theologiae, I.103.6)

To put it another way, according to Aquinas, it is a greater perfection, and therefore is more fitting, for God to share His causality with His creatures, making them authentic causes that can cause by their own natures along with God, than for God to remain the sole cause acting within the universe.

By creating through evolution, God is able to invite His creatures to work with Him to generate the novelty and diversity of life. As Aquinas noted, this is a greater perfection than if He had chosen to create life on His own via special creation.

Second, according to Aquinas, God also created the diversity of creatures because no single creature can adequately reflect the perfection of God:

We must say that the distinction and multitude of things come from the intention of the first agent, who is God. For He brought things into being in order that His goodness might be communicated to creatures, and be represented by them; and because His goodness could not be adequately represented by one creature alone, He produced many and diverse creatures, that what was wanting to one in the representation of the divine goodness might be supplied by another. For goodness, which in God is simple and uniform, in creatures is manifold and divided and hence the whole universe together participates in the divine goodness more perfectly, and represents it better than any single creature whatever. (Summa theologiae, I.47.1)

Therefore, in my 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. Four billion species created over a three-billion-year period is far more than the eight million extant species today. In fact, it would have been ecologically impossible for all four billion species to coexist on our planet, because there is only a limited number of ecological niches on the planet at any given moment in time.

If they had been created together, for instance, the large carnivorous dinosaur, Tyrannosaurus rex, would have wiped out the Asian elephant, Elephas maximus. However, with evolution—and not with special creation—these natural kinds were able to exist at separate moments in history to uniquely manifest the glory of their Creator.

To sum up, why did God choose to work via an evolutionary process rather than will a special creation? Because it better reveals His glory and His power. Because it reveals better that He is God.

Fr. Chaberek claims that an evolutionary view of creation would undermine God’s intent to create a world with varying degrees of perfection because each natural kind “requires continual change toward greater perfection in ‘struggle for life’ and ‘survival of the fittest.’” But I do not think that an evolutionary biologist today would claim that evolution is a movement toward greater perfection. The dean of evolutionary biologists, the late Stephen Jay Gould, certainly denied this explicitly in his book, Full House. Instead, he claimed that evolution is a movement toward a greater diversity of life. And as I noted above, this would not be contrary to God’s intent of creating a material world that is ordered towards His glory.

The increase in the number and kinds of organisms over evolutionary history better reflects the infinite ways in which finite creatures could reflect the infinite beauty of God. Evolution does not undermine divine providence.