One finds the phrase cognitio fidei in Thomas Aquinas and in such Thomist writers as Josef Pieper. It translates as 'knowledge of faith.' The genitive is to be interpreted subjectively, not objectively: faith is not the object of knowledge; faith is a form or type of knowledge. But how can faith be a type of knowledge? One ought to find this puzzling.
On a standard analysis of 'knows,' where propositional knowledge is at issue, subject S knows that p just in case (i) S believes that p; (ii) S is justified in believing that p; and (iii) p is true. This piece of epistemological boilerplate is the starting-point for much of the arcana (Gettier counterexamples, etc.) of contemporary epistemology. But its pedigree is ancient, to be found in Plato's Theaetetus.
It is obvious that on the standard analysis mere belief is inferior to knowledge since if I believe what is false I don't have knowledge, and if I believe what is true without justification I don't have knowledge either. How then can mere belief be a form or type of knowledge? It is rather a necessary but not sufficient condition of knowledge. Or so it seems to the modern mind.
Another puzzle has to do with certainty. Whether or not knowledge entails certainty, it seems to the modern mind that belief definitely does not entail certainty: what I believe but do not know I cannot be certain about since if I believe but do not know, then either truth is lacking or justification is lacking or both. How then can mere belief be said to be certain? And yet we read in Aquinas that "It is part of the concept of belief itself that man is certain of that in which he believes." (Quoted from Pieper, Belief and Faith, p. 15).
Is easy to understand how one who believes but does not know that p can be subjectively certain that p; but it is difficult to understand how such a person can be objectively certain that p. Objective certainty, however, alone has epistemic value.
We now turn to the remarkable Edith Stein (1891-1942), brilliant Jewish student of and assistant to Edmund Husserl, philosopher, Roman Catholic convert, Carmelite nun, victim of the Holocaust at Auschwitz, and saint of the Roman Catholic church. In the 1920s Stein composed an imaginary dialogue between her two philosophical masters, Husserl and Aquinas. Part of what she has them discussing is the nature of faith.
One issue is whether faith gives us access to truth. Stein has Thomas say:
. . . faith is a way to truth. Indeed, in the first place it is a way to truths — plural — which would otherwise be closed to us, and in the second place it is the surest way to truth. For there is no greater certainty than that of faith . . . . (Edith Stein, Knowledge and Faith, tr. Redmond, ICS Publications 2000, pp. 16-17)
Now comes an important question. What is it that we as philosophers want? We want the ultimate truths about the ultimate matters. If so, it is arguable that we should take these truths from whatever source offers them to us even if the source is not narrowly philosophical. We should not say: I will accept only those truths that can be certified by (natural) reason, but rather all truths whether certified by reason or 'certified' by faith. Thus Stein has Aquinas say:
If faith makes accessible truths unattainable by any other means, philosophy, for one thing, cannot forego them without renouncing its universal claim to truth. [. . .] One consequence, then, is a material dependence of philosophy on faith.
Then too, if faith affords the highest certainty attainable by the human mind, and if philosophy claims to bestow the highest certainty, then philosophy must make the certainty of faith its own. It does so first by absorbing the truths of faith, and further by using them as the final criterion by which to gauge all other truths. Hence, a second consequence is a formal dependence of philosophy on faith. (17-18)
But of course this cannot go unchallenged by Husserl. So Stein has him say:
. . . if faith is the final criterion of all other truth, what is the criterion of faith itself? What guarantees that the certainty of my faith is genuine? (20)
Or in terms of of the distinction made above between subjective and objective certainty: what guarantees that the certainty of faith is objective and not merely subjective? The faiths of Jew, Christian, and Muslim are all different. How can the Christian be sure that the revelation he takes on faith has not been superseded by the revelation the Muslim takes on faith?
Stein's Thomas replies to Husserl as follows:
Probably my best answer is that faith is its own guarantee. I could also say that God, who has given us the revelation, vouches for its truth. But this would only be the other side of the same coin. For if we took the two as separate facts, we would fall into a circulus vitiosus [vicious circle], since God is after all what we become certain about in faith. [. . .]
All we can do is point out that for the believer such is the certainty of faith that it relativizes all other certainty, and that he can but give up any supposed knowledge which contradicts his faith. The unique certitude of faith is a gift of grace. It is up to the understanding and will to draw the practical consequences therefrom. Constructing a philosophy on faith belongs to the theoretical consequences. (20-22)
So there you have it. There are two opposing conceptions of philosophy, one based on the autonomy of reason, the other willing to sacrifice the autonomy of reason for the sake of truths which cannot be certified by reason but which are provided by faith in revelation. It looks as if one must simply decide which of these two conceptions to adopt, and that the decision cannot be justified by (natural) reason.
My task, in this and in related posts, is first and foremost to set forth the problems as clearly as I can. Anyone who thinks this problem has an easy solution does not understand it. It is part of the tension between Athens and Jerusalem.