The literature about evolution in biology amounts to thousands of books and papers published every year. There is also a significantly smaller number of publications by Thomists who try to show how evolutionary theory may be reconciled with Thomistic philosophy. But the common problem of both Thomistic and non-Thomistic literature is a lack of clear and consistent definitions of terms. Let’s look at just a couple of examples found in the Thomistic literature.

One of the books says: “The task of evolutionary biology is to explain new species as arising out of earlier ones in the same way that mountains and lakes are explained as having arisen out of earlier geological formations.”

Here biological evolution is compared to geological processes such as elevation of mountains, erosion of rocks, etc. But the first part defines evolution as new species arising from the earlier species. We already have confusion, because a new species is definitely something entirely new compared to an old species. For example, the common evolutionary story says that birds evolved from reptiles, thus, initially there was only reptiles (mostly robust and heavy quadrupeds walking firm on the earth) and finally we have small, gentle and quite agile creatures that differ in almost everything from the reptiles (internal and external organs and systems in birds are completely different from those in reptiles). Thus in the course of time we would see the emergence of some total novelties – like wings, pneumatic skeletons or the “four-cycle” respiratory system. We do not see such creativity in any geological processes. What we see instead is mountains changing into different mountains or into valleys or sea beds which change into mountains or valleys again. Nothing new is created in geological processes. So, what does the definition tell us? Does it mean that those new “species” that evolve from the old ones are just minor variants emerging due to environmental adaptations? We know that if we move a population of foxes from dry and hot conditions to cold and humid they may survive and adapt by developing thicker fur, thicker blood and perhaps other features. But we do not see an emergence of a new species from the earlier species. In fact we know quite well that a polar fox is as much a fox as a desert one. Moreover, when we check the fossil record we do not see any dramatic changes in the forms of animals that inhabited the earth over millions of years. A hundred million years old crocodile fossil is identical to the currently living crocodile of the same species. Surely, some species appear, another disappear in the fossil record, but we do not have evidence that those “newer ones” evolved from the older ones. On the contrary, forms of all species remain unchanged over millions of years, and this is the rule rather than an exception. So, how should we understand the quoted definition? It seems like it is at least incoherent. It claims that entirely new species of living beings may arise owing to evolution, but at the same time it compares evolution to geological processes which do not create anything new. Moreover, this definition assumes the existence of species – new species evolve from the earlier ones. But where did those earlier come from? According to this definition evolution does not explain the origin of species. At the end of the day, we do not know if the definition speaks about macro or micro evolution. Does it mean that evolution creates species, or just minor variants within species? Are all species sharing common ancestry, or there were many beginnings of life? These and many other questions remain unanswered. But the answer to these questions is crucial to know what we are talking about when discussing evolution and Aquinas’s philosophy.

Another definition found in Thomistic literature says: [Evolutionary theory are] the scientific claims that all the living organisms on our planet have a common biological origin and that these diverse organisms arose through a process of natural selection acting on genetic diversity.”

Also this definition assumes what it wants to explain and thus ends up in circular reasoning. If natural selection acts on genetic diversity the genetic diversity must exist prior to natural selection. But genetic diversity is carried by the diversity of organisms which, according to the definition, are produced by natural selection. The diversity of life must exist in order that natural selection creates the… diversity of life. One could also wonder how much science there is in the “scientific claim” that “all the living organisms on our planet have a common biological origin”. Such a general statement about all life sounds more like a philosophical postulate rather than a conclusion from any scientific research.

Given those ambiguities and even contradictions in the definitions provided by Thomistic evolutionists we need to resort to clear and consistent definitions and – by doing so – define what the problem of Aquinas and evolution is about.

Biological evolution

Evolution may be considered as a process occurring either in nature or in culture. In culture it is evolution of laws, languages, customs, arts, political systems, etc. In sum, evolution in culture applies to the changes in the sphere created by rational and typically human activity. And this is not the type of evolution we are concerned with. In nature we can speak about evolution of cosmos (cosmic evolution) which says about changes of the planetary systems, emergence of stars etc. In physics evolution applies to changes in particles, in chemistry evolution refers to changes in compounds. Chemical and biochemical evolution is usually understood as the theory attempting to explain the origin of life. But in the center of our interest is evolution in biology. Biological evolution is about changes in living beings. However, not all changes are controversial and not all biological changes are an object of the hot debate over evolution. If evolution means just “change over time” it does not create much controversy. Everybody can see that living beings change – they are born, mature, grow old and die. New generations come. We can see that ecosystems and populations change too. Living beings migrate, sometimes they die out, sometimes propagate beyond the capacity of their habitats. Moreover, individuals differ from each other and very often one trait (biological characteristic) may dominate a population while another trait may completely disappear. We also see environmental adaptations of living beings due to the changes in natural conditions. But none of these is the point of controversy. At least, it is not a point of controversy when we ask the question of whether Thomistic philosophy is compatible with biological evolution or not. The controversy enters the debate when we make a distinction between micro and macroevolution.


Microevolution is a process of changes that is quite well documented by biological sciences. It involves adaptive changes in living beings that may happen because of various factors. These may include genetic mutations. One example is the resistance of bacteria to antibiotics which is produced through random mutations and natural selection. This kind of evolution does not impact the external form of an organism in any way. Another example is an adaptation of organisms to different environments. This kind of change may impact quite dramatically an external look of an organism. It may, for example, produce different color of skin or fur, longer fur or debilitated fur, it may influence blood density, weight and size of an organism, and many other features. But the common point of all microevolutionary changes (whether internal or external) is that they never produce a new natural species. Instead, these changes occurring in individuals fall within the limits of their species.


Macroevolution consists of three grand claims.

First, that all organisms share common ancestry, i.e., all species whether still alive or extinct branched out from one or a few ancestors in a distant past. This also means that all living beings are connected by the process of natural biological generation.

Second, transformation of species into another species is possible by means of natural generation. This means that an individual or a population of one type of organism can (and in fact will) change into a completely different type of organism given enough time and other natural conditions (such as selective pressure, mutation, propagation, etc.).

The third grand claim is that those changes that produced all diversity of life were natural, i.e., they happened thanks to the properties and laws inherently present in nature. No supernatural power worked in this process.

All the three claims are an object of the ongoing controversy. The problem of compatibility of Thomistic philosophy with biological macroevolution boils down to the three questions: Is Thomism compatible with universal common ancestry? Is Thomism compatible with the transformation of species? Can new species emerge naturally?


Evolutionists (such as Darwin and his followers) deny the existence of species or maintain that it is impossible to define species. But if this was the case, how could a book on the origin of species (including Darwin’s own work) make any sense? In fact evolutionists are incoherent in their adoption of species. Darwin had to deny the existence of species in order to introduce the idea of transformation of species. This is why he made such bold claims like “No line of demarcation can be drawn between species”. But in the course of his work he changes his mind without admitting it. He says that there are “true species” and “intermediate species”. Those “intermediate” are just the initial stages of the production of those “true” or “distinct” species. This kind of confusion entered biology and will remain there as long as macroevolutionary theories will drive the interpretation of data in biology. It is impossible to claim transformation of species and to maintain a clear definition of species. But Aristotle and Aquinas did not doubt that species exist and that we can define them. Moreover, if evolutionists postulate that species arise from one another and all species share one ancestor they need to have some definition of species. To avoid confusion introduced by Darwin we will distinguish four ways of understanding species.

1) Logical species—the idea of species taken as a logical subcategory of the broader category of genus. In this sense, species can be attributed to individuals quite arbitrarily, simply by projecting new working definitions on different classes of beings. Species, taken in merely a logical sense, is a relative term that simply maintains a distinction between classes and sub-classes of a specified group of objects or organisms. Nominalists, those claim that species do not exist, understand them only in this way.

            2) Metaphysical species—a species predicated with respect to a substantial form. Metaphysical species includes beings that have the same substantial form.

            3) Natural species—natural kinds of living organisms, such as dogs, cats, cows and horses. Natural species can be defined according to the three levels of the human knowledge:

  1. a) In natural science (biology), a natural species are living beings belonging to the same genus or family according to the classical taxonomy.
  2. b) In philosophy, a natural species includes organisms that share the same nature. In this context “nature” is defined by Aquinas as “the essence of a thing as it is ordered to the proper operation.” From the metaphysical perspective, natural species can be seen as living composites of form and matter that share the same substantial form.
  3. c) In theology natural species may be identified with kinds mentioned in Genesis 1.

            4) Biological species (or modern scientific notion of species)—according to one modern definition by Ernst Mayr, a biological species signifies all populations in which individuals are prospectively able to interbreed in their natural environment and produce fertile offspring.

Only the third understanding of species (as natural species) is relevant in the controversy over evolution. In fact, biological macroevolution assumes that new natural species (not biological species) can emerge from the previous ones. This also helps settle the difference between micro and macroevolution. Microevolution is all changes occuring within natural species whereas macroevolution postulates transformation and emergence of new natural species. Additionally, natural species is defined according to the three levels of the human knowledge. This means that no matter whether we approach evolution from theological, philosophical or scientific perspective the problem is always the same: Can new natural species emerge through evolution? And this is the matter of the debate.

Biological macroevolution and Darwinism

We said before that biological macroevolution consists of the three grand claims about the biological realm. All three tell us “what would happen if evolution worked long enough”. These three grand claims describe the supposed effects of evolution rather than a mechanism or the way how evolution happens. We should therefore distinguish between the supposed effects of macroevolution and the biological mechanism that is supposed to produce those effects. The commonly adopted mechanism of evolution in biology is the Neo-Darwinian one. It consists of two basic factors – random genetic mutations and natural selection. Mutations provide diversity from which selection chooses the “better adapted” traits and cancels the “worse adapted”. Only the mechanism of evolution is a proper object of scientific scrutiny. The grand claims remain just the grand claims which are not so much scientific as philosophical. The grand claims have never been demonstrated according to the methods of natural sciences. Thus, it is important to keep in mind the distinction between the mechanism and the effects of evolution. The mechanisms may change, but the supposed effects (the three grand claims) remain the same from the times of Darwin to now. In our debate between Thomism and evolution we are not concerned with the mechanism of evolution but with the three grand claims. Consequently, it does not matter for us how much credibility any mechanism gains in science, whether it is confirmed or disproved. Darwinism or Neo-Darwinism may be true in what it claims about the changes of species and at the same time the three grand claims about the origin of species may be completely false.

Theistic evolution

Biological macroevolution is a scientific theory aimed at explaining the origin of species. Many Christian scholars after Darwin asked if biological macroevolution is compatible with the Christian faith. Those who answered in positive never challenged the theory. Instead, they came up with the idea that God used evolution to produce species. Evolution was called a secondary cause of creation: As a carpenter uses saw and hammer to make a bench God used evolution to create the biodiversity. Those scholars often distinguish between the mechanism of evolution and the effects. They say that the mechanism may change, but the grand evolutionary claims about universal common ancestry and natural transformation of species remain valid. This is how biological macroevolution becomes theistic evolution, that is, the theological concept saying that God did not create species separately by his supernatural power but instead used the powers of nature such as generation, random variation and natural selection. In theistic evolution there is room for God – He is considered the highest Being who acts as the final cause of the evolutionary process and also as the primary efficient cause. Theistic evolutionists claim to be radically different from atheistic evolutionists who deny any higher cause or any finality in evolution. However, theistic evolutionists agree with atheists that divine causality left no marks in the natural order. Consequently science of biology can fully explain the origin of species. Thomistic evolutionists say the same what theistic evolutionists with the addition that theistic evolution is compatible with Thomistic philosophy. According to Thomists (those who support theistic evolution), Aquinas would not have a problem with admitting the three grand claims of biological macroevolution: universal common ancestry, transformation of species and the natural origin of species.


Aquinas defines creation as not a change, but a simple emanation of being out of nothing. A thing may start to exist in two ways: either by creation or by a change, such as: mutation, generation or any type of movement. But creation has no movement. It is not a process. It is simple emanation of a being which is produced according to its entire substance. Hence three characteristics of creation: (1) It is always a supernatural (and direct) act of God. (2) It is instant and not continuous (not a process). (3) It does not involve any secondary causes. Evolution, in contrast, is: (1) natural, (2) a process (it is a change) and (3) continuous, i.e., extended in time. Moreover, creation is a theological notion whereas evolution a scientific one. We see therefore that creation and evolution are mutually exclusive – either something is created or evolved. Creation and evolution are in logical opposition. Even so, many theologians believe that one and the same thing can come about by creation and by evolution. One influential theologian complains about “somewhat fierce debate raging between so-called creationism and evolutionism, presented as though they were mutually exclusive alternatives” … “This antithesis is absurd” – according to the theologian. Another renowned theologian believes that “the alternative theory of evolution or creationism is too simplistic, but the dispute on creation and evolution constitutes a challenge that touches upon important ethical issues…”. If we adopt the classic Christian understanding of creation and the classic scientific understanding of evolution it is not absurd to oppose evolution and creation. Rather it is absurd to say there is no opposition between them.

It is not rare that Thomistic evolutionists call creative act an intervention. They usually claim that God does not “intervene” in the natural world, instead He guides everything using natural secondary causes. By identifying creation with an intervention they dismiss creation as interventionism. There is a lot of confusion in this claim. Intervention from the Latin inter-venio means that God enters a chain of causes and effects in order to change its course and bring about a different effect that would otherwise follow. But in creation there is no previous chain. God begins being in an absolute way whether it is a being of the total universe (first creation out of nothing) or the subsequent creation of particular new substances. Intervention presupposes the existence of causes and effects, creation does not presuppose anything. Creation is not intervention. We can compare divine creative activity to a painter adding a new object to a painting. It does not destroy anything that was already painted. It does not break any of the existing order. It simply adds something entirely new. In creation an absolute novelty is produced. Nature, however, is not capable of producing an absolute novelty.

Another confusion regarding the term “creation” is apparent in a phrase like “the mode of creation”. Thomistic evolutionists use this phrase to explain how God created species. They say that the Bible tells us that species were created, but it does not tell us anything about how it happened. To explain how species begun to exist belongs to science. But this is already contradiction in terms, because if God created something there cannot be any process to account for its origin. Saying “a mode of creation” is redundant because creation signifies both that a thing began to exist and how it began to exist, namely, out of nothing by the direct act of God.