Aquinas’ Program: Is Natural Theology a Science?

It is an important distinction to make between natural theology and revealed theology, since the contrast is sharp but very much required. Natural theology is the “practice of philosophically reflecting on the existence and nature of God independent of real or apparent divine revelation or Scripture” [1]. Revealed theology in contrast, does operates within a context of divine revelation. As Charles Taliaferro briefly explains:

[R]evealed theology may take as authoritative certain New Testament claims about Jesus and then construct a philosophical or theological model for understanding how Jesus may be human and divine. Natural theology, on the other hand, develops arguments about God based on existence of the cosmos, the very concept of God, and different views of the nature of the cosmos, such as its ostensible order and value. [2]

Here I wish to examine an understanding of natural theology that sees Aristotelian philosophy as an impetus for not only the creation of scholastic theology, but how this philosophy gave new direction to the discipline of theology altogether. If you look to Diogenes Allen’s work on this subject [3], the best way to see this value is through the “program of Thomas Aquinas.” Particularly, viewing Aquinas’ Five Ways as inherently demonstrative and hence in this way makes natural theology a science.

Essence and Existence

St. Anselm in his Proslogion (ch. 2) argues for the existence of God from the very concept of God. Otherwise stated, if we can know what God is then we can know that God is; i.e., from God’s essence we can conclude God’s existence. Aquinas however, disagreed. He believed that since we have no direct or immediate access to the divine essence, we can’t deduce from this alone that God exists. Hence, according to Aquinas, God’s existence is not self-evident and we thus need a demonstrative proof in its place.

Allen (p. 138) has an interesting way of showing this. Aquinas’ argument that God’s existence is not self-evident is sort of like lacking a middle term in Aristotelian logic. For instance, consider this argument:

  1. All x‘s are y’s.
  2. P is an x.
  3. Therefore, p is a y.

In this example, x is the middle term. In relation to our discussion, if we knew what causes existence to belong to God then we would have the middle term. In the proposition “God exists” we do not know what connects the predicate (exists) to its subject (God). The obvious reason for this is that God’s essence is unknowable to us. Therefore, it is a legitimate pursuit for Aquinas to try and make a proof establishing God’s existence.


The Five Ways

Although Aquinas believes that we cannot grasp or comprehend God’s essence, we can nonetheless know some [true] things about God’s existence via our rational capacities. For instance, Aquinas in his First Way draws upon the Aristotelian doctrine of motion as the actualization of potency. He adapts Aristotle’s argument for the Prime Mover towards just one being who is Pure Act, with no potentiality. However, Aquinas goes further than Aristotle (but doesn’t go away) in suggesting that God is the efficient cause, so that all other efficient causes are merely dependent upon God’s efficient causality (Second Way).

In his Third Way Aquinas suggests that God’s essence is not distinct from his existence, which explains why all other beings are distinct in their essence from their existence. This is what categorizes God as a necessary being, who must exist. Indeed, God by virtue of his very nature cannot not exist. As Allen writes, “God’s essence is a pure, subsistent act of existence. God is sheer existence, the only being who can be said to be existence” [4]. This is furthermore what philosophers mean when they employ that Latin phrase, esse per se: “God is being in God’s self” [5].

Lastly, Aquinas even in his Commentary on the First Book of the Sentences of Peter Lombard argues that whatever is the conclusion of a demonstration is not known per se notum (known intuitively); philosophers have demonstrated the existence of God, therefore the existence of God is not per se notum. [6]



Knowledge of not only God but even his attributes (intellect, will, goodness, etc.) are the product of demonstration, and it is for this reason why natural theology is understood as a science. This program in natural theology by Aquinas was inspired by the metaphysics of Aristotle, in that First Philosophy (as he called it) had a concern for the study of being qua being.

In other words, the primary (or highest) form of reality is substance – such as individual substances in the sum total of being – and philosophy should furthermore concern itself with those substances that are free of change and are pure actualities. This form of reality should have very tight eyes on it from First Philosophy, which was the goal of Aristotle and was extended in the form of Christian theology by Aquinas.




  • [1] Charles Taliaferro, “The Project of Natural Theology,” The Blackwell Companion to Natural Theology. ed. William Lane Craig and J.P. Moreland (Blackwell Publishers: 2010), p. 1.
  • [2] Ibid.
  • [3] Diogenes Allen, Philosophy for Understanding Theology (John Knox Press: 1985), ch. 6.
  • [4] Ibid., p. 140.
  • [5] Ibid.
  • [6] See Distinction III, Question 1, Article 2.

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