“The aim of natural science is not simply to accept the statements of others, but to investigate the causes that are at work in nature.”


St. Albertus Magnus¹

In Aristotle’s epistemology, the goal of scientific enquiry is knowledge of the human person. This epistemology and the natural science that springs from it point beyond the material world to a realm of essences. Unlike Plato’s epistemology, Aristotle’s synthesis of material and spiritual realities allows theologians a greater framework in which to practice their discipline, interpreting revelation in light of the full range of human experience. St. Thomas Aquinas, the Angelic Doctor, took full advantage of this synthesis and of natural science in his Christology.
Natural science, as practiced by Aristotle, aids theology in its ends; it provides a necessary foundation and allows reasonable abstractions from that foundation. As the Philosopher explains in Physics,

The natural way of doing this is to start from the things which are more knowable and clear to us and to proceed towards those which are clearer and more knowable by nature; for the same things are not knowable relatively to us and knowable without qualification. So we must follow this method and advance from what is more obscure by nature, but clearer to us, towards what is more clear and more knowable by nature.²

The Angelic Doctor refers to the same process as the “order of determination.” As McInerny and O’Callaghan explain,

The first purchase on natural things is via “physical object” or “natural thing.” The “order of demonstration” involves finding the properties of things as known through this general concept. Then, specifying the subject further, one seeks properties of things known through the less common concepts.³

The scientist asks general questions, then increasingly more specific questions about the object of inquiry. He starts by asking whether something exists, then what that thing is, then what properties it possesses, and finally why it possesses those properties.

St. Thomas puts this method to use in his proofs of God’s existence. Before he attempts to determine what God is, he determines that God exists and notes as proof the changing condition of the material universe, the notion of efficient cause or material being, the difference between possibility and necessity in being, the gradation of qualities in things, and the governance or design of things in the material world.⁴ While the third and fourth are more abstract proofs, the first, second, and fifth proofs are clearly based on sensory experience of the material world. This foundation is critical for all other arguments he makes concerning the qualities of God, including the Triune nature of God and the Incarnation.

St. Thomas’s Christology continues with his discussion of the fitness of the Incarnation. His initial affirmation is, if nothing else, a Christian recasting of Aristotelian epistemology:

On the contrary, It[sic] would seem most fitting that by visible things the invisible things of God should be known, for to this end the whole world was made, as is clear from the word of the Apostle (Rom. I. 20): “For the invisible things of God… are clearly seen, being understood by the things that are made.”⁵

In other words, through our sensory experience, we come to know about the material world and, by analyzing our experience with our intellect, come to understand those things that are immaterial. St. Thomas then addresses the relationship of human and divine natures in the hypostasis.

According to Aquinas’s third proof, God is a necessary being, while man and other material entities are contingent beings. We can imagine the possibility for a man or any other material being not to exist. The very possibility of imagining it means that at one point the being did not exist. If this premise is true, then it would follow that at some point nothing existed. If nothing existed at one time, nothing could have come to exist and be in existence now. Therefore, there must be some being in whom essence (nature) and existence are the same, in which existence is necessary.⁶ This being is that Being whom we call God.

In the Incarnation, we have a Divine hypostasis,⁷ the Son. The concept of the Divine hypostasis, or Divine person, is analogous to the human person. Personhood is that combination of a rational nature and existence, or as Aquinas describes, “a subsistent individual of a rational nature.”⁸ However, the human person, as a contingent being, has essence and separate existence; whereas with God, we have three Divine persons, all of whom are one necessary being and in whom essence is not distinct from existence. In Summa Theologica, St. Thomas repeatedly uses such analogical arguments from the material world to describe the qualities of God or the relationships among the three persons of the Godhead. As with all analogies, the similarities among the entities compared are as important as the differences.

Divine Being is necessary and, as such, immutable.⁹ One of the arguments St. Thomas counters in Summa Theologica is the idea that the Divine and human natures became a single nature. He notes, on the contrary, that combining two dissimilar things results in changes to both. However, the immutability of Christ’s Divine nature renders such a change impossible. St. Thomas necessarily concludes that the union of natures must occur in the person of Christ rather than the natures.¹⁰ From here, the Divinity of that person must follow. The Divine essence (nature) and existence of Christ are inseparable and immutable. Because the “nature… designates the species,”¹¹ it follows that the hypostasis, the person of Christ, is Divine. The human nature, being separate and distinct from its existence, has the same subsistence in the Divine person.

In addition to the Divinity of Christ, St. Thomas also addresses the humanity of Christ. In Aristotle, we see that a person gains, through sensory perception and its analysis, not only a sense of the material world, but also a sense of some capability unique to humans that is distinct from or transcends the material, the soul. In explaining the immaterial nature of the soul, Aquinas disputes the belief that souls are bodies, holding up the distinction between animate and inanimate bodies, as well as the idea that a body requires something immaterial (the soul) to be its mover or principle of life. This soul is immaterial or incorporeal and it is also subsistent.¹² Man comprises body (matter) and soul. Again, to make this argument, St. Thomas resorts to observable data and sensory experience to show that the soul cannot be material and that soul and body are united.

At this point, we are left with one problem: what to do with the human nature of Christ. If we see this as a leftover human nature, excluded from the hypostasis because of the union of essence and existence in the Son, then we have forgotten the important difference between the Divine person and the human person, the point at which the analogy of personhood breaks down: necessary as opposed to contingent being. As Jack Bonsor explains,

The distinction between essence and existence, between abstraction and judgment, makes it possible to grasp Thomas’ point. One must think of existence as “happening” or “occurring” as an event.¹³

Christ’s Divine nature is one and the same as His existence and is outside of time and space. It happened in the past, is happening now, and continues to happen. Christ’s human nature is distinct from His human existence, which is an event in fixed time and in location.

As Pope John Paul II stressed in Fides et Ratio, reason and faith are allies, not enemies. The philosophy of St. Thomas, with its reliance on Aristotelian epistemology and natural science, exemplify this alliance. The strength of Aristotle’s epistemology is its balance, its middle road, its golden mean. Aristotle and Aquinas both recognized the interplay and interdependence of material and immaterial reality. While Platonism might allow theologians to plumb the depths (or perhaps soar to the heights) of spiritual reality, it consigns the material world to a weak similitude of that reality and reduces sensory experience to shadow play on the wall of a dimly lit cave. The Angelic Doctor demonstrates that, through the use of natural science together with faith, one can illuminate the cave, the cistern, and all other recesses in which the Truth might be hidden.

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Works Consulted

Aristotle. Physics. Trans. R. P. Hardie and R. K. Gaye, The Internet Classics Archive, Ed. Daniel C. Stevenson, 1994–2000, .

Aquinas, Thomas, Saint. Great Books of the Western World: The Summa Theologica of Saint Thomas Aquinas. Ed. Robert Maynard Hutchins. Vols. 19–20. Chicago: Encyclopædia Britannica, Inc., 1952.

Bonsor, Jack A. Athens & Jerusalem: the Role of Philosophy in Theology. Eugene: Wipf and Stock Publishers, 2003.

Kennedy, D. J. “Albertus Magnus.” Catholic Encyclopedia. Robert Appleton Company, 1907. Online Edition. K. Knight, 2005 .

McInerny, Ralph, and O’Callaghan, John. “Saint Thomas Aquinas,” The Stanford Encyclopedia of Philosophy. Ed. Edward N. Zalta. Spring 2005 ed. .