Thomistic Natural Theology
For Thomists, Natural Theology is the study of what can be known about God apart from revelation. It tries to show that certain truths about God (e.g. that God exists, that there is only one God, that He is Good) are demonstrable by reason. In a sense, the things even belonging to Natural Theology are contained in Scripture insofar as Scripture speaks of many things which could be discovered by humans without God revealing them.
Sacred doctrine essentially treats of God viewed as the highest cause, for it treats of Him not only so far as He can be known through creatures just as philosophers knew Him — “That which is known of God is manifest in them” (Rom. I. 19)– but also so far as He is known to Himself alone and revealed to others.S.T. Ia q. 1, a. 6.
Thus, for Aquinas, on the one hand, there are things that God has revealed about Himself which could be known by reason alone (Natural Theology), and on the other hand, there are things that He Himself alone knows about Himself, which He reveals to others, and which are, and must always be in this life, objects of religious belief (Sacred Doctrine). The light of reason can provide the warrant for holding the former as true; only the authority of God provides the the warrant for believing the latter. Implicit in this distinction is the understanding that the same thing cannot be the object of knowledge and belief at the same time. If one knows, i.e. has discovered by the use of natural reason, that God exist, then one cannot at the same time believe this truth as something that God has revealed. Aquinas calls what reason can know about God, i.e. the object of Natural Theology, preambles to faith:
The existence of God and other like truths about God, which can be known by natural reason, are not articles of faith, but are preambles to the articles; for faith presupposes natural knowledge, even as grace presupposes nature and perfection the perfectible.(S.T. Ia, q. 2, a. 2)
Aquinas’ notion of preambles to the faith, however, does not mean that believers first prove that there is a God and that He is one, etc., and then come to assent to things which He has revealed about himself. Believers generally don’t distinguish between the sorts of theology to which the truths contained in Scripture belong to. Especially among whose who had their faith taught to them from infancy, believers generally accept that there is a God, and that He is One, for instance, precisely because it is part of the faith that they have had all of their lives, which faith also tells them that the Son of God became man. Knowledge of the preambles, then, are not necessary for one to be a believer. Nor are they sufficient.
…when natural theology is successful it does not provide any grounds for faith in any strict sense of grounds. That is, if natural theology succeeds in its initial task, to prove the existence of God, no de fide truth follows from this as a consequence. If it did, the de fide truth would be transformed into a known truth ….Ralph McInerny, “On Behalf of Natural Theology,” in Being and Predication (Washington, DC: The Catholic University of America Press, 1986), p. 251.
The notion of preambles to faith assumes that there are some truths which can only be known as a result of God’s revelation, which no amount of human reasoning could ever begin to penetrate. To claim that there are preambles to faith, i.e. things about God which human reasoning can penetrate, does not change this basic assumption. Thus, the notion of preambles to faith precludes even the possibility that one can prove the truth of, e.g. the Catholic Faith. It also precludes the possibility that a non-believer must (by logical necessity), on the basis of Natural Theology, become a believer. “The fact is that no one became a believer as the result of an argument. Of course God could use the occasion of philosophizing to give one the grace of faith, but then He can use any occasion He wishes.”(“On Behalf of Natural Theology,” p. 254) But the contention that there are preambles to the faith does means that something that has been revealed could in principle be proved and thus known.
On the other hand, it may appear that one who has religious faith that God exists and has revealed Himself to humankind must either (temporarily) abandon that faith in order to engage in Natural Theology, or do so insincerely, as though already assured of the conclusions of her proofs. That is, the possibility of engaging in Natural Theology, e.g. of proving the existence of God, does not seem to be open to a believer since, being a believer, she already holds as true what a proof is supposed to allow one to conclude, i.e. “that God exists.” Since a proof is a sort of discovery, it seems that in order to sincerely engage in a proof, one cannot already hold that the conclusion is true.
…(I)t seems to follow that any believer who sets about formulating a proof for the existence of God is in a position of having to say, “I don’t know whether God exists.” But that seems to amount to a denial of the existence of God or at least a denial of the truth of the proposition “God exists.” Fortunately, this is not the case. To believe that God exists is not to know that God exists. The believer can ask himself if the proposition which he holds to be true on God’s say-so could be held on some other basis, say that of a proof. While he is pursuing this possibility, he does not cease to hold that the proposition “God exists” is true, though his warrant for its truth is authority, revelation. The denial of “I know God to exist” is not equivalent to a denial of “God exists.” The truth of the proposition must be held in some way or other, by faith or knowledge. While one cannot hold it by faith and knowledge at the same time, one need never stop holding it to be true (by faith) while one looks to see if it can be known to be true (by a proof).“On Behalf of Natural Theology,” p. 253.
Consequently, for a Christian to engage in Natural Theology, thereby claiming that it is possible to prove that God exists and is one, does not diminish the fact that it is still necessary to believe (with religious faith) in Christ, the Trinity, the Resurrection, and the Forgiveness of Sins. By the same token, the acceptance of the conclusions of Natural Theology does not entail or necessitate religious faith.
In fact, Aquinas generally believed that the prospects were pretty poor for unaided human reason to achieve very much success even in its own sphere of Natural Theology.
For truth about God, such as reason can know it, would only be known by a few, and that after a long time, and with the admixture of many errors.S.T. Ia, q. 1, a. 1.
The pursuit of Natural Theology, while not directly leading to religious faith, nevertheless is of benefit to believers in their faith. First of all, it gives one better insight into nature of revelation, for it shows to what extent the things contained in Scripture could have been discovered by human reason. For the Christian this really shows how little knowledge about God can be gained apart from His self-disclosure, and how much grace is given as an utter mystery. The Christian can, thus, marvel at how much more God reveals (that He is Three Persons, that the Son became Man to save us from our sins, that He gives us the promise of the Resurrection) than we would ever have suspected had He not told us.
Ultimately, however, the fact that natural theology is possible is itself a matter of belief. Thomas, like most medievals, was fond of noting the fact that God has revealed through St. Paul that “The invisible things of God are clearly seen, being understood from the creation of the world through things that are made, both his eternal power and divinity” (Romans I, 20). Thus, it appears, God believes that there are things about Himself which can be known by unaided human reason. “And yet, involved in our faith is the tenet that, apart from faith, the God who has revealed himself to man, though not as he has revealed himself, can be known by man.” (McInerny, “Philosophizing in Faith,” in Being and Predication, p. 245). Thus, the believer might engage in Natural Theology in order to do what Scripture says can be done.
If Natural Theology gives any help to supporting the truth of faith, and does not simply benefit from the wider and deeper perspective of Sacred Theology, it is that Natural Theology can remove impediments to the faith. For if it can show that the reasoning of those who oppose the faith are false, then it can at least show that the Christian is not unreasonable in believing the things that God has revealed. This indirect aid of Natural Theology to the believer serves to remind her that the things she believes with religious faith are concerned with the truth, and could be falsified if the unbeliever proved God did not exist.
Indeed, we can find here one of the motives for the interest the community of believers has always taken in the task of natural theology. It is not that a proof of God’s existence is direct support for truths of faith in the strong sense. But if God can be known to exist, one impediment to accepting that He has revealed truths about Himself is removed. Nor could the community of believers ignore claims that it is nonsense to assert that there is a God. (Truths of the faith) cannot be derived from truths known about God but, negatively, if it were known that there is not a God, (truths of the faith) would eo ipso be destroyed. A God known not to exist cannot reveal truths about Himself.McInerny, “St. Bonaventure and St. Thomas,” in Being and Predication, pp. 156-7.
Thus, by proving that there is a God, that He is unique, good, all- powerful, etc., the believer can at least silence those who would attack the very possibility of what she believes.