We proceed to the eighth article thus:
1. It seems that sacred doctrine does not proceed by argument. For Ambrose says: "where faith is sought, eschew arguments" (De Fid. Cath.), and it is especially faith that is sought in this doctrine. As it is said in John 20:31: "these are written, that ye might believe." It follows that sacred doctrine does not proceed by argument.
2. Again, if sacred doctrine proceeded by argument, it would argue either on the ground of authority or on the ground of reason. But to argue from authority would be beneath its dignity, since "authority is the weakest kind of proof," as Boethius says (Topica 6), and to argue by reason would be unworthy of its end, since "faith has no merit when human reason proves it by test," as Gregory says (Hom. in Evang.26). It follows that sacred doctrine does not proceed by argument.
On the other hand: Titus 1:9 says of a bishop, "holding fast the faithful word as he hath been taught, that he may be able by sound doctrine both to exhort and to convince the gainsayers."
I answer: just as other sciences do not argue to prove their own principles, but argue from their principles to prove other things which the sciences include, so neither does this doctrine argue to prove its principles, which are the articles of faith, but argues from these to prove other things. Thus in 1 Cor.15 the apostle argues from the resurrection of Christ to prove the general resurrection. We must remember, however, that the inferior philosophical sciences do not prove their own principles, nor defend them against one who denies them. They leave this to a higher science. The highest of them, metaphysics, does argue in defence of its principles, provided that he who denies them concedes anything at all. But it cannot argue with him if he concedes nothing, although it can refute his reasoning. Now sacred doctrine, which has no superior, likewise argues at times with one who denies its principles, provided that its adversary concedes something of what is received through revelation. Thus we argue from the authority of sacred doctrine against heretics, and from the authority of one article of faith against those who deny another. But when an adversary believes nothing at all of what has been revealed, there is no way of proving the articles of faith by argument, except by disproving any grounds which he may bring against the faith. For since faith takes its stand on infallible truth, the contrary of which cannot possibly be demonstrated, it is obvious that proofs cited against the faith are not demonstrative, but answerable.
On the first point: although arguments of human reason cannot suffice to prove matters of faith, sacred doctrine argues from the articles of faith to other things, as said above.
On the second point: proof by authority is especially characteristic of this science, because its principles are obtained through revelation. The authority of those who received revelation has to be believed. But this does not detract from the dignity of the science. Appeal to an authority which depends on human reason is the weakest kind of proof. Appeal to an authority founded on divine revelation is the most telling. Yet sacred doctrine does make use of human reason, not indeed to prove the faith (which would take away its merit), but to clarify certain points of doctrine. Since grace does not supplant nature, but perfects it, reason ought to be the servant of faith in the same way as the natural inclination of the will is the servant of charity -- "bringing into captivity every thought to the obedience of Christ," as the apostle says in II Cor.10:5. Sacred doctrine uses even the authority of philosophers in this way, wherever they have been able to know the truth through natural reason. In Acts 17:28, for example, Paul quotes the words of Aratus: "as certain also of your poets have said, For we also are his offspring." Sacred doctrine uses such authorities, however, as supporting and probable arguments. It uses the canonical Scriptures as the proper authority from which it is bound to argue, and uses other teachers of the Church as authorities from which one may indeed argue with propriety, yet only with probability.