It is a rule that the principles which make something are also the principles for repairing it. If a house falls down, it is restored according to the plan by which it was first made.
Aquinas, De Rationibus ch. 5
Aquinas notes at the beginning of De Rationibus that his aim is not to attempt to prove Christian faith by necessary reasons, stipulating that the Christian faith cannot be reduced to a series of incontestable propositions because there are certain elements of faith that transcend the mind and can only be known to be true because they are revealed by God. It would seem that the natural inclination of a person reading this text would be to quickly dismiss parts of Christian belief that can only be known through revelation as inadmissible as evidence in dialogue. After all, Aquinas himself states that “it would be useless to quote passages of Scripture against those who do not accept its authority” (De Rationibus, Ch. 1).
Here Aquinas makes two stipulations: the first principles of a defense cannot be based exclusively in reason because certain thing can only be known by faith through revelation, and dialogue between Christians and Muslims must use terms that are held in common by both. It would at first glance appear impossible to craft an argument that does not run afoul of one of these two stipulations. These limiting factors, however, do not preclude the possibility of making an exclusively rational argument based on one revealed truth. This is exactly how Aquinas constructs his defense of the doctrine of the Trinity. Beginning with the shared belief that God created the world according to a revealed account, Aquinas proceeds to defend the doctrine of the Trinity in strictly rational terms with the only necessary point of revelation being the belief that God created the world, establishing the uniqueness and oneness of the Son with the Father and the relationship of the Spirit to both strictly from the use of reason.
It appears, then, that Aquinas thinks that the truth of Christian faith can be demonstrated (as opposed to proven) if one were to accept just a few points of revelation. The idea that these few points of revelation are shared in common with Muslims has profound impact on his view of interreligious dialogue. It would seem that for Aquinas, Muslims can even arrive at belief in the doctrine of the Trinity (which Aquinas sees as the the principal doctrine in Christianity) through the use of reason, implying that if one were to concede the doctrine of creation of the world by God, it would therefore be most reasonable to believe as Christians do about God, for “whatever comes from the Supreme Truth cannot be false, and what is not false cannot be repudiated by any necessary reason” (2).
Paul Elhallal 
Ah. I should have looked at your post before replying to your reply on mine. We should keep talking about this, as I’m willing to be proven wrong here, but I don’t think that Aquinas is saying quite what you have him saying here, i.e. that trinitarian doctrine can follow simply from a belief that God created the world. If memory serves, for Thomas, I think the belief that God created all things would be considered something that natural theology could attain, i.e. not a specifically revealed truth of Christianity. The revealed truths that he is speaking of are things like the doctrine of a trinity of Persons, which, I think, he would say must have the clarity of the New Testament (and perhaps the following counsels?). The project he takes up in the pages we looked at is assuming the New Testament and showing that it is not rationally impossible. There would be nothing stopping him or some other creative thinker, I think, from building a similar argument for the existence of, say, four persons in the godhead simply from a belief that God created the world. His argument is not foolproof or necessary; it is, as he says often, “fitting.” I haven’t explained this all that well; we should talk more in class.
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Thanks Paul! I hadn’t seen the importance of the notion of creation to Thomas’ method of reasoning in this treatise. There’s probably some other points in his reasoning which he assumes Muslims will agree to, such as the goodness and power of God.
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