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Evolution presents two general problems: transformation from one species to another species, and progress to a higher species. St. Thomas had an answer to both problems. First, St. Thomas endorses secondary causality. He believed that God is a Creator, but uses secondary causes to providently rule the world. Second, St. Thomas endorses degrees of service. Aristotle and Aquinas observed that less noble creatures are in service if more noble creatures. Therefore, St. Thomas gives the general principles for the solution of the general problems with evolution.

Does St. Thomas believe in creation by God? Yes, he does. Creation is the action by which God gives existence to the universe by drawing it from nothing. This is taught in Sacred Scripture (Genesis 1: 1 et seq.). St. Augustine, who was the deepest patristic thinker on creation, wrote, "The creator is only the one who produces things as the first cause" (Augustine De Trinitate 1. 3. 9. 18).²⁶ The teaching of St. Thomas follows Augustine.²⁷ St. Thomas teaches, "Creation is the production of every thing in its entire substance which has no created or uncreated presupposit" (Aquinas Summa Theologiae 1. 63. 3).²⁸ St. Thomas believes that God is the creator. He teaches, "The more universal an effect, so much more elevated is its proper cause, because when higher is the cause, so greater are the effects over which it extends its power...Now existence is more universal than becoming, even pertaining to things which are not mobile like rocks and the like, as even philosophers note...It happens then that above the cause that only moves or changes, there exist a cause which is the first principle of being and this is none other than the Subsistent Being Himself," (Aquinas Summa Contra Gentiles 2. 16).²⁹

Does St. Thomas endorse secondary causes? Yes, he does, and this is the key to the current thesis on evolution. Extrinsicism in the opponents of the existence of secondary causes is found in Plato with his Ideas, Avicenna with his Separated Intelligence, and Ibn Gabirol with the external divine will; in these cases, the secondary cause would do nothing at all because it was receiving everything from the outside. Intrinsicism in the other major opponent of the existence of secondary causes is found in Anaxagoras, who holds that the various physical, intellectual and moral operations are already performed and realized virtually from within; in this case the effects that secondary causes seem to produce are already virtually realized either in a cause itself or in others.³⁰ St. Augustine and St. Thomas affirm the absolute primacy of God as the principle cause of everything produced by nature. St. Thomas affirms there are secondary causes in nature. As proof of secondary causes, St. Thomas has three arguments (Aquinas Summa Theologiae 1. 105. 5).³¹ First, without secondary causes there would be no connection for creatures between their causation and the effect; creatures would be impotent and their powers in vain. Second, every being exists through its operations, so that without secondary causality, creatures existence would be imperiled. Third, less perfect things are ordered to more perfect: matter is ordered to form as the first act, and matter is ordered to operation as the second act, in such a way that operation is the goal of created things. Therefore, St. Thomas confers upon secondary causes the full share of being and efficacy to which they are due. In the real world, the nature of the effect is similar to the nature of the cause, so that warmth does not chill, and humans generate humans. So the existence of natural laws suppose that God created beings endowed with causality.³² How can the same effect be produced by two different causes (God and the natural agent) at the same time? These causes are at the same time, but not under the same relation, e.g., a workman uses an axe to cut wood, and both are causes. The analogy applies to God, but God’s influence on the secondary cause penetrates more deeply, so that when God grants existence, God grants form, movement, and efficacy.³³ Thus the existence of secondary causes points to no lack of power in God, but to the immensity of God’s goodness (confer: Aquinas Summa Contra Gentiles 1. 13).³⁴ Philosophically, secondary causality in creatures is an affirmation of the principle of causality which is fundamental to classical metaphysics and especially to Thomistic metaphysics. The principle of causality regulates the relationship between cause and effect according to the definition of Aristotle, which St. Thomas made his own, declaring, "Everything that is moved is moved by another," or in another way, "Everything that happens presupposes a principle that produces it."³⁵

Does St. Thomas teach that lower creatures are in service of higher creatures? Yes, he does, and so reprises Aristotle who taught that "nature proceeds little by little from things lifeless to animal life" and "there is observed in plants a continuous scale of ascent toward the animal."³⁶ This observation of natural ascent is not only helpful here for the proof of evolution in general, but is also the key to our next thesis on the finality of the evolutionary process, or evolutionary progress. There are two helpful texts from Aquinas. St. Thomas notes, "...less noble creatures are in the service of the more noble...Further, every creature is in the service of the perfection of the universe...Finally, the totality of the universe with all its parts is ordered to God as its goal" (Aquinas Summa Theologiae 1. 65. 2).³⁷ The second text is the classic text on the evolutionary view of Aquinas, cited by Benignus, Mondin and Maritain:³⁸

But since, as was already stated, everything which undergoes motion tends as such toward a divine likeness in order to be perfect in itself, and since a thing is perfect in so far as it becomes actual, it follows that the intention of everything that is in potentiality is to tend to actuality by way of movement. Hence the more final and the more perfect an act is, the more is the appetite of matter inclined to it. Therefore the appetite whereby matter seeks a form must tend toward the last and most perfect act to which matter can attain, as to the ultimate end of generation. Now certain grades are to be found in the acts of forms. For primary matter is in potentiality, first of all, to the elemental form. While under the elemental form, it is in potentiality to the form of a compound; wherefore elements are the matter of a compound. Considered under the form of a compound, it is in potentiality to a vegetative soul; for the act of such a body is a soul. Again the vegetative soul is in potentiality to the sensitive, and the sensitive to the intellective. This is shown in the process of generation, for first in generation is the fetus living a plant life, afterwards the life of an animal, and finally the life of man. After this no later or more noble form is to be found in things that are generated and corrupted. Therefore, the last end of all generation is the human soul. Consequently, the elements are for the sake of compounds, the compounds for the sake of living things, and of these plants are for the sake of animals, and animals for the sake of man. Therefore, man is the end of all generation. (Aquinas Summa Contra Gentiles 3. 22)³⁹

This text from St. Thomas’ third book of the Summa Contra Gentiles, chapter 22, deserves a most serious comparison with the theory of evolution, considering just the elements involved in both. Both Darwin and St. Thomas begin by observation, Aquinas beginning by observation of "motion." Second, both are evolutionary, at least in the wide sense, Aquinas considering everything undergoes motion "in order to be perfect." Thirdly, both appear universal, Aquinas considering "everything." Fourth, both process of evolution and the view of Aquinas are by "generation." Fifth, by creaturely generation, neither Darwin nor Aquinas treat the first creation, which does not happen by generation, but is from noting. Sixth, neither Darwin nor St. Thomas ignore matter, Aquinas noting "matter inclined." Seventh, both Darwin and St. Thomas see evolutionary tendencies in nature itself, but Aquinas differs by noting the "appetite of matter" itself. Eighth, evolution was ongoing, since Aquinas views matter as moving "towards the last and most perfect act." Ninth, a certain determinism is involved, since Aquinas notes "appetite...must tend." Tenth, both Darwin and St. Thomas deal with species, since Aquinas notes "certain grades are to be found in the acts of forms." Eleventh, both Darwin and St. Thomas are dealing with substantial change, with Aquinas noting "elements...vegetative soul...sensitive (soul);...plant life...life of animal...life of man." Twelfth, some abiogenesis is indicated in Aquinas’ teaching: "...elements...compound...in potentiality to a vegetative soul." Thirteenth, the view of Aquinas here in a philosophic (not scientific) analysis tends toward a more monophylactic viewpoint, that "the appetite whereby matter seeks a form must tend toward the last and most perfect act to which matter can attain, as to the ultimate end of generation." Fourteenth, both Darwin and St. Thomas see evolution at least up to human kind, although Aquinas differs by noting that "man is the end of all generation." The comparison between Darwin, living in the nineteenth century, and St. Thomas, living in the twelfth century, should not be pushed too far, but the attempt to compare them is instructive.⁴⁰ The attempt at comparison is also complimentary to the genius of St. Thomas.

The way evolution can happen requires a limitation of form by matter for individuation into species. Substantial change must be possible for one species to change to another. Accidental change must perfect its subject and affect the whole being to bring about substantial change to a new species. St. Thomas affirms and explains material limitation of formal causality, substantial change, and that accidental change perfects the subject and affects the whole being. Therefore, St. Thomas gives the specific principles to explain evolution.

Does St. Thomas hold that species are educed from matter by the form? Yes, he does. According to St. Thomas there is a hierarchy among the causes: "Among the causes there exist the following order: the material is perfected by the formal cause, the formal by the agent cause, and the efficient by the final cause" (Aquinas Scriptum in Liber Sententiarum 4. 3. 1. 1. sol.1).⁴¹ It is the matter which limits and individuates the form, and "species" is an individuation. It is the form that gives matter "to be," actuates it, and by union with it makes the matter to be a body.⁴² However, St. Thomas does not view the form as necessarily simple, but says, "The more perfect form virtually contains whatever belongs to the inferior forms; therefore while remaining one and the same, it perfects matter according to the various degrees of perfection; for the same essential form makes man an actual being, a body, a living being, an animal and a man" (Aquinas Summa Theologiae 1. 76. 6. ad 1).⁴³ Note here that "man" is the species. The causality of these two principles is mutual, simultaneous and transcendental (the causality is intrinsic and mutual), and St. Thomas describes this as eduction from the matter, saying:

A thing naturally generated is properly said to be, since it has its ‘to be’ in its subsisting ‘to be’; form, however, cannot be said ‘to be’ in this way, since it neither subsists nor has its ‘to be’ of itself...Properly speaking, it is not the form which is, but form is that by which (something is)...This which is generated is not the form but the composit. And it is generated from matter, insofar as matter is in potency to the composit by being in potency to the form. Consquently, we cannot strictly say that the form is made in matter, but rather that it is educed from matter (Aquinas De Potentia 3. 8)⁴⁴

We cannot imagine such causality, for it transcends all sense experience, but intellectually we can understand why this must be.⁴⁵

Does St. Thomas affirm the possibility of evolutionary change, which would be the substantial change from one species to another species? Yes, he does make that affirmation, by affirming substantial change, by noting that even art can produce substantial change, and by affirming "privation" as an element of change. Can generation produce change that is either substantial (change between species) or accidental (change within species)? St. Thomas says:

Furthermore, since generation is movement toward form, corresponding to twofold form is twofold generation. Generation simpliciter (pure and simple) corresponds to substantial form, and generation secundum quid (relatively speaking) to accidental form. When a substantial form is introduced, we say that something comes into being simpliciter, as for example, man come into being or man is generated. But when an accidental form is introduced, we do not say that something comes into being simpliciter, but in this or that respect. Thus, when a man becomes white, it is not said absolutely that a man comes into being or is generated, but that he comes into being or is generated white" (Aquinas De Principiis Naturae, 6).⁴⁶

New substantial change can even be introduced by art, according to Aquinas, who teaches, "...nothing presents something to be made by art whose form...is a substantial form" (Aquinas Summa Theologiae 3. 75. 6. ad 1).⁴⁷ How does generation produce substantial change (between species) or accidental change (within species)? St. Thomas answers,⁴⁸ first, that privation is said only of a determined (apt) subject (Aquinas De Principiis Naturae 11), and secondly, that privation is the principle of becoming (Aquinas De Prinicipiis Naturae 12):

Further to be noted is that although generation is from nonexistence, we do not say that negation is the principle but privation is, because negation does not determine a subject. Non-seeing, for example, can be said even of non-beings, as we might say that a dragon (fabled monster) does not see, and we say the same of beings that are not fitted by nature to have sight, as stones. But privation is said only of a determined subject, in which, namely, a certain condition (habitus) is by nature more apt to come about; for instance, blindness is said only of things that are by nature apt to see. Moreover, generation does not arise from non-being simpliciter, but from the non-being that is in some subject, for example, fire does not arise from just any non-fire but from such non-fire as is apt to acquire the form of fire. And for this reason we say that privation is the principle, and not negation. (Aquinas De Principiis Naturae 11).

Privation, however, differs from other principles in that the others are principles of both existence and becoming. That a statue may be produced there must be bronze and, furthermore, there must be the shape of a statue. And when the statue exists, these two must exist. Privation, on the other hand, is only a principle of becoming and not of existing. For while a statue is in process it must not yet be a statue; if it were it could not come to be, because whatever comes to be, is not, except in successive realities, as time and motion. From the moment the statue exists there is no longer the privation of statue, since affirmation and negation cannot be simultaneous, and neither can privation and possession (habitus). Also, privation, as explained above, is a per accidens principle, but the other two (matter and form) are per se principles. (Aquinas De Principiis Naturae 12)

If substantial change is the true and real evolutionary change between species, why bother with accidental change? In fact, it appears that the being of accidents is the being of substance, as it were diffusing itself. It seems, therefore, that accidents are useless for the evolutionary process, since accidents are in the substance, and the substance is manifest and made known by the its accidents. St. Thomas confirms this by saying, "The emanation of proper accidents from the subject is not by way of transmutation, but by a certain natural result" (Aquinas Summa Theologiae 1. 77. 6 ad 3).⁴⁹ Again, St. Thomas confirms this apparent uselessness, saying, "There is always a proportion between the substance and its accidents" (Aquinas Scriptum in Liber Sententiarum 2. 27. 1. 6 ad 1). However, in the philosophy of nature, observation can confirm mutations in the subject, resulting in new accidents.⁵⁰ Mutations can happen because substance is a dynamic reality; and secondly, mutations are united to the subject and thus affect the whole being. St. Thomas notes how accidents integrate, determine and perfect the substance, even if the forms are accidental forms (confer: Aquinas De Malo 4. 2 ad 9). Every accidental change is somehow, at least mediately, an actuation of the substance, so the individual should never be considered as an immutable substance, but one constantly changing, constantly becoming. Secondly, St. Thomas notes that there is a unity, although an imperfect unity, of substance and accident, and consequent to this unity every accidental change must affect the whole being. Substance and accident have some unity since the nature of the accident is to be educed from the potency of the substance, and the accident naturally tends to inhere in the substance, yet substance and accident each have their individual "to be" so their union cannot be as close and intimate as between prime matter and substantial form. St. Thomas notes that the unity of a being depends on its "to be" (Aquinas Summa Theologiae 1. 11. 1 corpus). Since there is some unity, although an imperfect unity, of substance and accident, consequent to this unity every accidental change must affect the whole being.⁵¹ Renard notes, "At times accidental becoming prepares and disposes the substance, at least a longe, for such a change (generatio substantialis).⁵²

Does St. Thomas affirm active evolution? The modern question of evolution did not exist in the twelfth century when St. Thomas wrote. Nevertheless, there is an evocative statement of St. Thomas that God in the beginning creates all species together not in actual from but "in power and almost as in a seed" ("in virtute et quasi in semine," Aquinas Summa Theologiae 1. 66. 4) Such an affirmation of active evolution is confirmed by the opinion of St. Thomas that "Creatures, then, are clearly real causes not only of the ‘to be’ but of becoming" (Aquinas Summa Theologiae 1. 104. 1 corpus).