His ground: God's cooperation is simultaneous with our consent, so decree and foresight of our determination are conceived together
The basis of his view is this. On God's side, no working that precedes our will's consent imposes on the will a necessity to consent. The cooperation of God by which he brings about our will's very consent in no way precedes our will's determination; it exists together with it in time, in nature and in reason.
It follows that God's decree to bring about that consent cannot come before his foresight of the human will's determination. Things that exist wholly together in time cannot have an order of prior and posterior in God's foreknowledge and decrees.
From this he concludes that no free work of our will was predefined by God before he foresaw our will's free determination. Rather, the foresight of that determination and God's decree predefining our will's act must be conceived as together in God.
Latin
XXXVI. Fundamentum autem sententiae ejus est, quod ex parte Dei nulla operatio, quae voluntatis nostrae consensum antecedit, necessitatem imponit voluntati ut consentiat: illa vero Dei cooperatio, per quam efficit ipsum voluntatis nostrae consensum, nullo modo praecedit voluntatis nostrae determinationem, sed cum ea simul est tempore, natura, atque ratione. Unde sequitur decretum Dei de consensu illo operando, non posse esse prius praevisione determinationis voluntatis humanae: quia quae omnimodo simul sunt in tempore, non possunt ordinem prioris & posterioris habere in praescientia & decretis Divinis. Ex quibus concludit nullum opus liberum nostrae voluntatis praedefinitum a Deo fuisse, ante praevisam determinationem liberam nostrae voluntatis: sed determinationis illius praevisionem, & decretum Dei voluntatis nostrae actum praedefiniens, simul in Deo debere concipi.