Le Blanc: Vázquez is wrong; Durandus meant no grace through Christ at all
Anyone who reads Durandus carefully can see that Vázquez is wrong here. Durandus means that man, by the strength of nature and without any help of grace through Christ, can fulfil God's precepts. He proves his view with this argument: 'What man does naturally he can do of himself without grace. But the Gentiles, who do not have the law, do by nature the things of the law, as Romans 2 says. So the precepts of the law can be fulfilled without grace.'
So for Durandus to act without grace is the same as to act naturally and of oneself. But a man who does something through some help of grace from Christ cannot be said to do it naturally and of himself, even if he lacks habitual and justifying grace.
Besides, in question 2 of the same distinction Durandus raises against himself Christ's words, 'Without me you can do nothing.' He replies that they must be understood of God's general influence, by which he keeps the principles of things in being, and without which nothing would exist or happen.
Latin
XIII. Sed legenti & expendenti Durandum patet Vasquem in eo falli, & mentem Durandi esse, hominem per naturae vires, & absque auxilio quocunque gratiae per Christum implere posse divina praecepta. Etenim sententiam suam probat hoc argumento. Illud quod homo naturaliter facit, sine gratia per seipsum facere potest: sed gentes quae legem non habent, naturaliter quae legis sunt faciunt, ut dicitur, Rom. 2. Ergo praecepta legis sine gratia impleri possunt. Unde patet Durando idem esse sine gratia facere, quod naturaliter & per seipsum facere. Qui autem aliquid facit mediante aliquo gratiae per Christum auxilio, illud dici non potest naturaliter & per seipsum facere, etiamsi alioqui gratia habituali & justificante careat. Adde quod Durandus ad eandem distinctionem scribens quaestione secunda, objicit sibi illa Christi verba, sine me nihil potestis facere. Ad quae respondet verba ista intelligenda esse de generali influentia Dei, qua rerum principia in esse conservat, sine qua nihil esset, aut fieret.