Anselm of Canterbury, answered from the texts and cited to the page.
Theological argument operates in two distinguishable but related modes. Rationes necessariae — necessary reasons — are arguments cogent enough to compel rational assent independent of faith. The ontological argument, the necessity-of-incarnation argument, the necessary-being argument of Monologion 1-4 are intended as necessary reasons. Rationes congruentes or convenientiae — fitting or congruous reasons — are arguments that show a doctrine to be appropriate, consistent with the rest of what is known, fitting given the nature of God and the structure of creation, without strictly compelling assent.
The painter's analogy in Cur Deus Homo II.8 — that just as the cause of human condemnation had its origin in a woman, so it is fitting that the remedy for sin should be born from a woman — is a fitting reason rather than a necessary one.1 Both kinds of argument have their proper place; both contribute to intellectus fidei; but they should not be conflated.
Where a doctrine cannot be demonstrated by necessary reasons, fitting reasons may yet show it to be appropriate to what is otherwise believed.2345
And this also. If woman, whom God made from man alone, was made of a vir gin (de virgine}, it is peculiarly fitting for that man also, who shall spring from a woman, to be born of a woman without man. Of the pictures which can be superadded to this, showing that the God-man ought to be born of a virgin, we will say nothing. These are sufficient. Boso. They are certainly very beautiful and rea sonable.Proslogium, Monologium, Gaunilo's In Behalf of the Fool, and Cur Deus Homo, pp. 250–251
For, if he wished to lie, he would of course be able to; but, if he wished to avoid the other, he could no more do it than he could avoid being what he is. For he became man for this purpose, and it was on the faith of his coming death that he could re ceive birth from a virgin, as you said above. Anselm.Proslogium, Monologium, Gaunilo's In Behalf of the Fool, and Cur Deus Homo, pp. 271–272
For although they appeal to reason because they do not believe, but we, on the other hand, be cause we do believe; nevertheless, the thing sought is one and the same. And if you bring up anything in reply which sacred authority seems to oppose, let it be mine to urge this inconsistency until you dis prove it. Anselm. Speak on according to your pleasure.Proslogium, Monologium, Gaunilo's In Behalf of the Fool, and Cur Deus Homo, pp. 182–183
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
Operated with across the corpus from Monologion through De Concordia. I never fully systematized the distinction — the line between necessary and fitting reasons is sometimes clearer in my own argumentative practice than I explicitly mark it. McIntyre and Knowles have noted this unclarity. I think the distinction is doing real work even where its application is contestable: the painter's analogy of CDH II.8 is genuinely a different mode of argument from the unum argumentum of Proslogion 2 or the necessary-being demonstration of Monologion 3, and the difference matters for what the arguments can compel.