Jansenius, book 8, chapters 20-21: powers to opposites are not opposed
Thesis XLIV. He repeats and presses the same point in chapter 20 of the same book: 'We say that free choice, however strongly and effectively it has been anticipated and determined to do good by the delight of grace, can still not only fail to do the good but even do evil. This is true not in the composite sense, as the common phrase has it, but in the divided sense.
'At the very time when the will's choice is placed under the delight of grace that moves it effectively, and even while it is performing the good act of will, there is in that same will the power not to perform it, and even to sin. This is not because ceasing from the act it is then eliciting, or an actual sin, could coexist with the influence of delighting grace (which the composite sense would require), but because the power to cease and to sin can be joined with that same grace at the same time in the same choice of the will.
'Two contrary acts are opposed and cannot be in the same will at once; but powers for opposites are not opposed, either to one another or to the opposite acts, and they dwell together in the same subject at the same time, whether it is acting or at rest. So the will, however much it is captivated by the sweetness of grace, can refrain from doing what it is drawn to, since it keeps a real power of not acting even under the grace that draws it, although not acting itself cannot be combined with the working of grace in the same will at the same time.'
In the next chapter he puts it more briefly: 'The power of dissenting does not conflict with the actual motion of grace and with consent, although actual dissent cannot be joined with God's actual motion.'
Latin
XLIV. Quibus similia repetit & inculcat ejusdem libri capite vigesimo. Dicimus, inquit, liberum arbitrium quantumcunque vehementi atque efficaci gratiae delectatione praeventum atque determinatum ad faciendum bonum, adhuc tamen posse bonum non tantum non facere, sed etiam malum. Verum est istud non quidem in sensu composito, ut vulgo dici solet, sed in sensu diviso. Nimirum quia eodem tempore, quo voluntatis arbitrium sub gratiae delectatione efficaciter eam movente positum est, imo quo etiam actum voluntatis bonum facit, est in eadem voluntate potestas illud non faciendi, imo peccandi; non quod cessatio ab actu quem tunc elicit, aut actuale peccatum cum gratiae delectantis influxu consistere possit, (quod sensus compositus postularet) sed quia cessandi & peccandi potestas cum eadem gratia, simul in eodem voluntatis arbitrio conjungi potest. Nam quamvis duo actus contrarii sint oppositi, & in eadem voluntate simul esse non possint: potestates tamen ad opposita non sunt oppositae, nec sibi invicem, nec actibus oppositis, & in eodem simul subjecto, sive agente, sive quiescente commorantur. Sic ergo voluntas quantumcunque gratiae suavitate capiatur, potest non agere id quo rapitur, quia veram non agendi potestatem, etiam sub gratia rapiente retinet: quamvis fieri nequeat ut ipsa non actio, cum gratiae operatione in eadem simul voluntate copuletur. Quod capite sequenti paucioribus complectitur, Potentia quippe, inquit, dissentiendi non repugnat actuali gratiae motioni & consensui; quamvis fieri nequeat ut actualis dissensus, cum actuali Dei motione jungatur.