At the outset of Cur Deus Homo? Anselm cannot pull himself away from the beauty of the atonement. To say God humbled himself is not unsuitable (convenire) and makes no injury to God. It is perfectly appropriate, as evident from the symmetry of fall and redemption:
Death enters through disobedience, life through obedience; sin originated from a woman, the redeemer was born of a woman; man who fell at a tree is by a tree saved. It is indescribably beautiful (ineffabilem . . . pulchritudinem) that redemption should be accomplished hoc modo.
Boso is not convinced: Painters paint on solids, not on air or water. When Christians speak as Anselm speaks, unbelievers think they are merely painting on clouds. An aesthetic theory isn’t enough; some ratio must be invoked.
But Anselm won’t let go of the aesthetics just yet (he does eventually). It seems sufficient reason (satis necessaria ratio) that God would not leave mankind, his most precious handiwork (pretiosum opus eius) to be destroyed. It would not have been fitting if God had left humanity to be annihilated.
Whatever else Anselm says about the atonement, it is important to see that it begins as a rationalization of an utterly patristic aesthetic delight in the symmetry of Christ’s work.