Anselm of Canterbury, answered from the texts and cited to the page.
Faith and reason are not opposed but ordered. The right order, taught by Augustine and accepted by me, is that faith comes first — tempore auctoritas prior est, re autem ratio prior (in the order of time authority comes first, but in the order of reality reason is prior). Within faith, reason has two distinguishable but related functions.
For the believer, reason articulates the intellectus fidei, the understanding of what is already held — making explicit what is believed, drawing out its consistency, deepening the believer's grasp. For the unbeliever, reason can function apologetically: certain truths held by faith can be defended by rationes necessariae — necessary reasons that aim at compelling reasoned assent.
This is the explicit aim of Cur Deus Homo, which argues for the necessity of the incarnation "as if I had heard nothing of Christ — that is, that it is impossible for any man to be saved apart from Him." The same argument that strengthens the believer can address the unbeliever, because the necessary reasons are reasons, not partisan badges.12345
How with God there is neither necessity nor impos sibility 273 XVII16. How Christ's life is paid to God for the sins of men 279 XIX. How human salvation follows upon his death 283 XX. How great and how just is God's compassion 286 XXI. How it is impossible for the devil to be reconciled.. 286 XXII. How the truth of the Old and New Testament is shown in the things which have been said 287 ANSELM'S CUR DEUS HOMO.Proslogium, Monologium, Gaunilo's In Behalf of the Fool, and Cur Deus Homo, pp. 177–178
Anselm. Is it not sufficiently proved that man can be saved by Christ, when even infidels do not deny that man can be happy somehow, and it has been sufficiently shown that, leaving Christ out of view, no salvation can be found for man? For, either by Christ or by some one else can man be saved, or else not at all.Proslogium, Monologium, Gaunilo's In Behalf of the Fool, and Cur Deus Homo, pp. 238–239
In stating that the supreme Trinity may be said to consist of three substances, I have followed the Greeks, who acknowledge three substances in one Essence, in the same faith wherein we acknowledge three persons in one Substance. For they designate by the word substance that attribute of God which we designate by the word person.Proslogium, Monologium, Gaunilo's In Behalf of the Fool, and Cur Deus Homo, pp. 37–38
I request all who may wish to copy this book to prefix this brief preface, with the heads of the whole work, at its commencement; so that, into whosesoever hands it may fall, as he looks on the face of it, there may be nothing in the whole body of the work which shall escape his notice.Proslogium, Monologium, Gaunilo's In Behalf of the Fool, and Cur Deus Homo, pp. 178–179
And since investigations, which are carried on by question and answer, are thus made more plain to many, and especially to less quick minds, and on that account are more gratifying, I will take to argue with me one of those persons who agi tate this subject; one, who among the rest impels me more earnestly to it, so that in this way Boso may question and Anselm reply.Proslogium, Monologium, Gaunilo's In Behalf of the Fool, and Cur Deus Homo, pp. 179–180
Defended in Proslogion 1, Monologion preface, Cur Deus Homo I.1-3, and explicitly against Karl Barth's twentieth-century deflationary reading. The credo ut intelligam formula does not say that reason is suspended; it says that faith is the precondition of the kind of understanding I pursue. The same arguments — the ontological argument of Proslogion 2, the necessity-of- incarnation argument of Cur Deus Homo, the necessary-being arguments of Monologion 1-4 — function differently for different audiences without ceasing to be the same arguments. I never thought that the truths of faith could be discovered from reason alone, nor that they could be dispensed with once discovered. I do think that reason can go further within faith than the antidialecticians of my day allowed.