The Enchiridion 15
15 The Preceding Argument is in No Wise Inconsistent with the Saying of Our Lord: ‘A Good Tree Cannot Bring Forth Evil Fruit.’ PL 40, 238
But when we say that evil springs out of good, let it not be thought that this contradicts our Lord’s saying: “A good tree cannot bring forth evil fruit.” For, as He who is the Truth says, you cannot gather grapes of thorns, because grapes do not grow on thorns. But we see that on good soil both vines and thorns may be grown. And in the same way, just as an evil tree cannot bring forth good fruit, so an evil will cannot produce good works. But from the nature of man, which is good, may spring either a good or an evil will. And certainly there was at first no source from which an evil will could spring, except the nature of angel or of man, which was good. And our Lord Himself clearly shows this in the very same place where He speaks about the tree and its fruit. For He says: “Either make the tree good, and his fruit good; or else make the tree corrupt, and his fruit corrupt,”—clearly enough warning us that evil fruits do not grow on a good tree, nor good fruits on an evil tree; but that nevertheless the ground itself, by which He meant those whom He was then addressing, might grow either kind of trees.
Quomodo intelligendum illud, Non potest arbor bona, etc. Sed cum mala ex bonis orta esse dicimus, non putetur hoc dominicae sententiae refragari, qua dixit, Non potest arbor bona fructus malos facere. Non potest enim, sicut Veritas ait, colligi uva de spinis , quia non potest nasci uva de spinis; sed ex bona terra et vites nasci posse videmus et spinas. Et eodem modo tanquam arbor mala fructus bonos, id est, opera bona non potest facere voluntas mala: sed ex bona hominis natura oriri voluntas et bona potest et mala; nec fuit prorsus unde primitus oriretur voluntas mala, nisi ex angeli et hominis natura bona. Quod et ipse Dominus eodem loco, ubi de arbore et fructibus loquebatur, apertissime ostendit: ait enim, Aut facite arborem bonam et fructum ejus bonum, aut facite arborem malam et fructum ejus malum : satis admonens ex arbore quidem bona malos, aut ex mala bonos nasci fructus non posse; ex ipsa tamen terra cui loquebatur, utramque arborem posse.