Étienne Gilson says on page 83 of his Reason and Revelation in the Middle Ages that “faith is not a principle of philosophical knowledge, but it is a safe guide to rational truth and an infallible warning against philosophical error.” The point being that, while faith cannot provide rationally deductive demonstrations of this or that claim, it can provide the light and insight we need to direct our premises in rational investigation. We cannot philosophize by faith, but we can philosophize wrongly apart from faith. Since the content of faith, objectively given, is not an object of reason, it is not subject to purely rational strictures (much less to purely rational [i.e., deductive] demonstration, for which Anselm and Scotus argued in their ontological arguments). Because the content of faith is not an object of rational certainty, it is not an opinion at which we arrive, but is a testimony we accept as the Word of God. Moreover, because faith is not subject to rational demonstration, it is not arrived at by the intellect, but my a movement of the will, whereby the intellect arrives at truth it cannot grasp on its own without an elevating grace upon the pliant will.
This hold that faith places on philosophy has to do not with the supposed irrationality of faith claims, but with the very meaning of faith and reason as such. As soon as faith becomes an object of purely rational demonstration, it is eo ipso no longer an object of faith, and this in the same way your belief that I have brown hair is a ‘belief’ once you see (and thus know ‘scientifically’) that I do have brown hair. Accordingly, Gilson, citing St. Thomas in ST IIaIIae, q. 1, a. 5, notes that it is “impossible for one and the same thing to be an object of science [i.e., rational knowledge] and of belief for the same person…” (op. cit., p. 74). This disjunction is in order, since faith “implies assent of the intellect to that which the intellect cannot see [qua ‘scientific’ knowledge] to be true…” (ibid., p. 73). Further, Gilson argues, “if reason cannot prove them [i.e., dogmas] to be true, it cannot either prove them to be false” (ibid., p. 83). This is all of a piece with what we read in SCG I, 3:
Just as, therefore, it would he the height of folly for a simple person to assert that what a philosopher proposes is false on the ground that he himself cannot understand it, so (and even more so) it is the acme of stupidity for a man to suspect as false what is divinely revealed through the ministry of the angels simply because it cannot be investigated by reason.
As Gilson notes, St. Thomas’ anti-Averroist and, we might say, trans-Augustinian position on faith and reason stands in an interesting light, given the developments that ensued a few centuries after St. Thomas. For one thing, largely animated by Gehrard Groote, the Moderna devotio placed nearly all emphasis on our mystical perception of God, rather than any scholastic musings about Him. (Groote founded the fraternity of the Brethren of the Common Life in Deventer in 1381, and in 1475, a 12-year-old Desiderius Erasmus would enter the school for that fraternity.) This anti-scholastic, mystical attitude can be seen in Thomas à Kempis’ Imitatio Christi, as well as in the doctrine of Meister Eckhart (condemned in 1329 by Pope John XXII) about the soul’s union with God even this side of Heaven. It also finds expression in Luther’s excoriation of scholastic thought: “only without Aristotle can we become theologians.” (Cf. Gilson, Reason, pp. 86–94 for more details.) According to Ernst Cassirer, it also manifests in the development of Nicolas Cusanus’s thought. On page 13 in his The Individual and the Cosmos in Renaissance Philosophy, Cassirer states:
With the same assurance with which [Cusanus] denies the possibility of conceiving of the infinite by means of logical abstractions and generic concepts, he also denies the possibility of its conception through mere [mystical] feeling. In the mystical theology of the fifteenth century two fundamental tendencies stand sharply opposed to each other; the one bases itself on the intellect; the other considers the will to be the basic force and organ of union with God. In this dispute, Cusanus sides emphatically with the former. True love of God is amor Dei intellectualis: it includes knowledge as a necessary element and a necessary condition. No love can love what he has not, in some sense, known. Love by itself, without any admixture of knowledge, would be an impossibility. Whatever is loved is, by that very act, considered good; it is conceived of sub ratione boni. This knowledge of the good must spur on and give wings to the will, even though the What, i.e., the simple essence of the good in itself, remains inaccessible to knowledge. … The principle of docta ignorantia as ‘knowing ignorance’ re-affirms itself once again.
All of this suggests that there is a constant, inescapable tug on the rudder of Christian theology, which guides it back towards what Gilson calls the Augustinian family, a family characterized by St. Anselm’s credo ut intellegam. In that family, the will, drawn by love, always has an at least notional prominence over the intellective pursuit of truth that characterizes much of the Thomistic family. What I suggest for discussion is whether Cassirer underestimates the role of will in the scholastic account of our knowledge of God, since, as Gilson notes, where reason stops, grace must help the will to continue by faith. This point is made emphatically by James F. Ross in his papers dealing with cognitive voluntarism, available on his webpage).