Ord. II d. 34-37 qq.1-5, n. 46-47:
"Voluntas enim libera debitrix est ut omnem actum suum eliciat conformiter regulae superiori, videlicet secundum praeceptum divinum; et ideo quando agit difformiter ab ista regula, caret iustitia actuali debita (hoc est, iustitia quae deberet inesse actui et non inest): haec carentia, in quantum est actus voluntatis deficientis (sicut dicetur in aliqua solutione), est formaliter peccatum actuale."
Free will is a debtor, so that it elicits its every act in conformity to superior rules, namely, according to the divine commands. And therefore when it acts in a manner opposed to those rules, it lacks the actual requried justice (that is, the justice which ought to inhere or be present to the act and does not inhere). This lack, insofar as it as an act of the will turning aside is formally actual sin."