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The Treatise on Human Nature: Summa Theologiae 1a 75-89
This series offers central philosophical treatises of Aquinas in new, state-of-the-art translations distinguished by their accuracy and use of clear and nontechnical modern vocabulary. Annotation and commentary accessible to undergraduates make the series an ideal vehicle for the study of Aquinas by readers approaching him from a variety of backgrounds and interests.
432 pages, Paperback
Published November 15, 2002
About the author
Thomas Aquinas
2,816 books1,239 followersPhilosophy of Saint Thomas Aquinas, a Dominican friar and theologian of Italy and the most influential thinker of the medieval period, combined doctrine of Aristotle and elements of Neoplatonism, a system that Plotinus and his successors developed and based on that of Plato, within a context of Christian thought; his works include the
Summa contra gentiles
(1259-1264) and the
Summa theologiae
or theologica (1266-1273).
Saint Albertus Magnus taught Saint Thomas Aquinas.
People ably note this priest, sometimes styled of Aquin or Aquino, as a scholastic. The Roman Catholic tradition honors him as a "doctor of the Church."
Aquinas lived at a critical juncture of western culture when the arrival of the Aristotelian corpus in Latin translation reopened the question of the relation between faith and reason, calling into question the modus vivendi that obtained for centuries. This crisis flared just as people founded universities. Thomas after early studies at Montecassino moved to the University of Naples, where he met members of the new Dominican order. At Naples too, Thomas first extended contact with the new learning. He joined the Dominican order and then went north to study with Albertus Magnus, author of a paraphrase of the Aristotelian corpus. Thomas completed his studies at the University of Paris, formed out the monastic schools on the left bank and the cathedral school at Notre Dame. In two stints as a regent master, Thomas defended the mendicant orders and of greater historical importance countered both the interpretations of Averroës of Aristotle and the Franciscan tendency to reject Greek philosophy. The result, a new modus vivendi between faith and philosophy, survived until the rise of the new physics. The Catholic Church over the centuries regularly and consistently reaffirmed the central importance of work of Thomas for understanding its teachings concerning the Christian revelation, and his close textual commentaries on Aristotle represent a cultural resource, now receiving increased recognition.
Saint Albertus Magnus taught Saint Thomas Aquinas.
People ably note this priest, sometimes styled of Aquin or Aquino, as a scholastic. The Roman Catholic tradition honors him as a "doctor of the Church."
Aquinas lived at a critical juncture of western culture when the arrival of the Aristotelian corpus in Latin translation reopened the question of the relation between faith and reason, calling into question the modus vivendi that obtained for centuries. This crisis flared just as people founded universities. Thomas after early studies at Montecassino moved to the University of Naples, where he met members of the new Dominican order. At Naples too, Thomas first extended contact with the new learning. He joined the Dominican order and then went north to study with Albertus Magnus, author of a paraphrase of the Aristotelian corpus. Thomas completed his studies at the University of Paris, formed out the monastic schools on the left bank and the cathedral school at Notre Dame. In two stints as a regent master, Thomas defended the mendicant orders and of greater historical importance countered both the interpretations of Averroës of Aristotle and the Franciscan tendency to reject Greek philosophy. The result, a new modus vivendi between faith and philosophy, survived until the rise of the new physics. The Catholic Church over the centuries regularly and consistently reaffirmed the central importance of work of Thomas for understanding its teachings concerning the Christian revelation, and his close textual commentaries on Aristotle represent a cultural resource, now receiving increased recognition.
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November 16, 2025
The Treatise on Human Nature is a section of Thomas Aquinas’ Summa Theologia that spans Questions 75 through 89, though the edition I read includes only up to Question 86. The treatise mainly focuses on on the soul: its essence, its powers or capacities and its operation. In this review, I will discuss the sections and ideas that caught my interest, and then offer some brief opinions on the work as a whole.
Upon reading the very first question, the reader encounters a perhaps puzzling statement:
In [ST I.76.4], Aquinas questions if there are any other substantial forms in the human body. Objection four raises an interesting idea: if the human body is made of elements such as air, fire, and so on, in what Aquinas calls a mixed body, then the forms of these elements would seem to remain in the body, implying that there are substantial forms besides the soul. The reply focuses on the notion that these elements are not present in the body actually, but rather virtually (or in potentiality). This means that, despite their substantial forms being gone, their properties remain through the power of the higher form, the soul, which actualizes the elements. His reply showcases once again the notion of substantial holism, in which beings exist as unified wholes rather than mere collections of other substances.
Another idea that ties to the soul’s simplicity and unity of being is that it is whole not only in the whole body, but also in each part of it [ST I.76.8]. You may think that because the eye, a body part, has its own power to see, there must be a corresponding part of the soul that enables this function. However, as it becomes clear in later Questions, Aquinas follows Aristotle in believing that the soul is one and simple, yet possesses many capacities. One such capacity is the power of sense perception, which enables the soul to receive and process sensory information, including vision. Despite its various operations, these capacities are not separate parts of the soul, they all belong to one unified whole, acting as the substantial form of the entire human substance.
This brings us to the division of the soul into capacities, dispositions and acts. Capacities are the soul’s innate ability to perform specific kinds of operations. They are abilities that stem from the very essence of the soul, such as the sensitive, intellective and vegetative capacities. Dispositions, on the other hand, are natural inclinations that enable the soul’s capacities to act. They are tendencies that make the soul ready to execute what it is capable of . At last, acts are the actual execution of these powers. As an example, consider the soul’s capacity for virtue. Through repeated virtuous acts, the soul acquires the disposition for courage. Through this disposition, the will is then inclined to further virtuous acts, which links Aquinas once again to Aristotle, particularly his Nicomachean Ethics.
Next, let us consider how a person may sensorily perceive and grasp the ulterior species of something: that is, let us grasp Aquinas’ epistemology. How may we understand a dog?
The author says:
But how can the we get from the senses to the intellect? If we can only grasp particulars, such as the dog, how can the intellect understand universals, such as animal? The solution is in the existence of two intellects. The possible intellect passively acts as a blank slate, being in potentiality to knowledge. However, as it is passive, it needs an active counterpart that imprints the forms upon it, and that is the agent intellect. This second intellect acts on the concrete image of the dog, abstracting away its particularities, such that what remains is the general nature common to all dogs: the actually intelligible species (for the full discussion, reference [ST 1.84] and [ST 1.85]). Thus, Aquinas enables science and knowledge through the the examination of the universal nature existing in the particular, shying away from the skepticism that hounded future epistemology.
A potential problem arises: we seem to grasp what is common to individuals before grasping the particular. If grasping the particular counts as the actuality of knowledge, and not grasping anything represents potentiality, insofar as it has the potential to understand, grasping the genus is an intermediate state between the two. Therefore, our knowledge moves from the more general to the more particular. I question: if knowledge of the particular truly is the actualization of the intellect, how can this actualization ever be wrong? What if we are mistaken about the very same particular? Further: what is useful as knowledge seems to be the general nature of something. How can we consider this most useful knowledge as correct if by its very definition it is not fully actualized?
A distinction needs to be made regarding what is primary in the order or being and what is primary in the order of knowledge and intelligibility. The particular is primary in the order of being, since it is a primary substance, subsistent and most real metaphysically. On the other hand, the universal is primary in the order of intelligibility. The intellect does not grasp the particular; as we’ve mentioned, it grasps the universal. Hence the universal is more intelligible to the intellect, and has a stronger explanatory power since it can explain an infinite number of particulars. In [ST 1.85.6], Aquinas argues that the intellect cannot err. Therefore, the grasping of the universal, which is the full and true act of intellect, is always correct, solving our later problem. Then, we’ve solved why knowledge of the universal is the most useful kind of knowledge, and why it can be correct. Now, it remains to explain the first question. Although this required some research, as this isn’t mentioned in the Treatise, the author understands that knowing a particular is not a simple act of apprehension. It is rather a composite act from both the intellect and the senses, where the judgment is tasked with linking both capacities. Consequently, it is neither the intellect nor the senses which err, it is the judgment.
As an addendum, Aquinas goes on a inquiry on will and the appetites (desires towards things that are considered good). From this whole affair, we take away that he acknowledges free will (liberum arbitrium), so no big disagreements or surprises there.
Unfortunately, the structure of the Summa once again was a detriment. I’ve come to suspect that this structure is actually inimical to my enjoyment. So, after the first few Questions, in order to break through what would be a slog, I decided to refrain from reading both the Objections and the Replies, keeping only with the Sed Contra and the Corpus. I understand that I have lost some useful material, but I must recognize that this strategy made the work at least twice as enjoyable. In a complementary capacity, there is always commentary to be read in order to fill the gap created by this scheme.
As a completely unrelated though interesting thought, I was thinking about how the word “actual” and its variants have mostly maintained their philosophical meaning in the English language, though in Portuguese this is not true. In my main language, “actual” may be translated as “na verdade” or “em si”. However, “atual” and “atualmente” denote a sense of temporality. This is so because, since Portuguese is a Romance language, it has suffered from the vulgarization of its Latin counterpart, while English borrowed the Latin term later, which contributed to mostly preserving its meaning in current times.
Upon reading the very first question, the reader encounters a perhaps puzzling statement:
Therefore, the nature of the human mind is not only non-bodily, but also a substance —That is, something subsistent.The soul being non-bodily checks out easily enough, but what about it being a substance? Aquinas famously adopts the Aristotelian hylomorphic view that substance is a composite of matter and form, so how can this quote fit within his own views? One needs to recall the theory developed on the work On Being and Essence, in which the soul is said to be a substance in a limited sense. The intellect has operations that do not use a bodily organ, therefore its principle (the rational soul) must have its own act of existence, surviving beyond the body’s death. The soul, then, is a substantial form, though not a full substance as its full operation needs a correspondent body, despite being subsistent.
In [ST I.76.4], Aquinas questions if there are any other substantial forms in the human body. Objection four raises an interesting idea: if the human body is made of elements such as air, fire, and so on, in what Aquinas calls a mixed body, then the forms of these elements would seem to remain in the body, implying that there are substantial forms besides the soul. The reply focuses on the notion that these elements are not present in the body actually, but rather virtually (or in potentiality). This means that, despite their substantial forms being gone, their properties remain through the power of the higher form, the soul, which actualizes the elements. His reply showcases once again the notion of substantial holism, in which beings exist as unified wholes rather than mere collections of other substances.
Another idea that ties to the soul’s simplicity and unity of being is that it is whole not only in the whole body, but also in each part of it [ST I.76.8]. You may think that because the eye, a body part, has its own power to see, there must be a corresponding part of the soul that enables this function. However, as it becomes clear in later Questions, Aquinas follows Aristotle in believing that the soul is one and simple, yet possesses many capacities. One such capacity is the power of sense perception, which enables the soul to receive and process sensory information, including vision. Despite its various operations, these capacities are not separate parts of the soul, they all belong to one unified whole, acting as the substantial form of the entire human substance.
This brings us to the division of the soul into capacities, dispositions and acts. Capacities are the soul’s innate ability to perform specific kinds of operations. They are abilities that stem from the very essence of the soul, such as the sensitive, intellective and vegetative capacities. Dispositions, on the other hand, are natural inclinations that enable the soul’s capacities to act. They are tendencies that make the soul ready to execute what it is capable of . At last, acts are the actual execution of these powers. As an example, consider the soul’s capacity for virtue. Through repeated virtuous acts, the soul acquires the disposition for courage. Through this disposition, the will is then inclined to further virtuous acts, which links Aquinas once again to Aristotle, particularly his Nicomachean Ethics.
Next, let us consider how a person may sensorily perceive and grasp the ulterior species of something: that is, let us grasp Aquinas’ epistemology. How may we understand a dog?
The author says:
Sensory operation requires a spiritual impression through which an intention of the sensible form is produced in the organ of sense.Through this, Aquinas means that the organ of sense, the eye, in order to see, must receive the intention of the object it is seeing. By intention, understand it as a formal likeness of an object, that serves as the medium for knowledge: it is a likeness of the object that modifies the medium (in our case, the air) according to its form, in order to point to the object being interacted with. Therefore, the dog modifies the medium between itself and the observer with its formal likeness, such that the eye may grasp its formal pattern.
But how can the we get from the senses to the intellect? If we can only grasp particulars, such as the dog, how can the intellect understand universals, such as animal? The solution is in the existence of two intellects. The possible intellect passively acts as a blank slate, being in potentiality to knowledge. However, as it is passive, it needs an active counterpart that imprints the forms upon it, and that is the agent intellect. This second intellect acts on the concrete image of the dog, abstracting away its particularities, such that what remains is the general nature common to all dogs: the actually intelligible species (for the full discussion, reference [ST 1.84] and [ST 1.85]). Thus, Aquinas enables science and knowledge through the the examination of the universal nature existing in the particular, shying away from the skepticism that hounded future epistemology.
A potential problem arises: we seem to grasp what is common to individuals before grasping the particular. If grasping the particular counts as the actuality of knowledge, and not grasping anything represents potentiality, insofar as it has the potential to understand, grasping the genus is an intermediate state between the two. Therefore, our knowledge moves from the more general to the more particular. I question: if knowledge of the particular truly is the actualization of the intellect, how can this actualization ever be wrong? What if we are mistaken about the very same particular? Further: what is useful as knowledge seems to be the general nature of something. How can we consider this most useful knowledge as correct if by its very definition it is not fully actualized?
A distinction needs to be made regarding what is primary in the order or being and what is primary in the order of knowledge and intelligibility. The particular is primary in the order of being, since it is a primary substance, subsistent and most real metaphysically. On the other hand, the universal is primary in the order of intelligibility. The intellect does not grasp the particular; as we’ve mentioned, it grasps the universal. Hence the universal is more intelligible to the intellect, and has a stronger explanatory power since it can explain an infinite number of particulars. In [ST 1.85.6], Aquinas argues that the intellect cannot err. Therefore, the grasping of the universal, which is the full and true act of intellect, is always correct, solving our later problem. Then, we’ve solved why knowledge of the universal is the most useful kind of knowledge, and why it can be correct. Now, it remains to explain the first question. Although this required some research, as this isn’t mentioned in the Treatise, the author understands that knowing a particular is not a simple act of apprehension. It is rather a composite act from both the intellect and the senses, where the judgment is tasked with linking both capacities. Consequently, it is neither the intellect nor the senses which err, it is the judgment.
As an addendum, Aquinas goes on a inquiry on will and the appetites (desires towards things that are considered good). From this whole affair, we take away that he acknowledges free will (liberum arbitrium), so no big disagreements or surprises there.
Unfortunately, the structure of the Summa once again was a detriment. I’ve come to suspect that this structure is actually inimical to my enjoyment. So, after the first few Questions, in order to break through what would be a slog, I decided to refrain from reading both the Objections and the Replies, keeping only with the Sed Contra and the Corpus. I understand that I have lost some useful material, but I must recognize that this strategy made the work at least twice as enjoyable. In a complementary capacity, there is always commentary to be read in order to fill the gap created by this scheme.
As a completely unrelated though interesting thought, I was thinking about how the word “actual” and its variants have mostly maintained their philosophical meaning in the English language, though in Portuguese this is not true. In my main language, “actual” may be translated as “na verdade” or “em si”. However, “atual” and “atualmente” denote a sense of temporality. This is so because, since Portuguese is a Romance language, it has suffered from the vulgarization of its Latin counterpart, while English borrowed the Latin term later, which contributed to mostly preserving its meaning in current times.
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September 22, 2012Very readable translation with extremely useful commentary. Pasnau's work is first-class.
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