Summa summae S. Thomae, Tomus II

I say 1°: Antecedent concupiscence rather causes a thing to be voluntary than involuntary¹. For a thing is said to be voluntary from the fact that the will is borne toward it. But through concupiscence the will is inclined to will what concupiscence desires. Therefore concupiscence contributes more to making a thing voluntary than involuntary.

I say 2°: Antecedent concupiscence increases absolute voluntariness, and diminishes perfect and free voluntariness.

The first part is proved. Absolute voluntariness is that which proceeds from an intrinsic principle with knowledge of the end materially, such as belongs to brutes. But antecedent concupiscence causes the will, with prior knowledge of the end materially, to be borne toward the object with greater force and inclination than if there were no concupiscence. Therefore. — The minor is proved: 1° By reason; for, concupiscence impelling, the intellect proposes to the will the object as more desirable, and therefore to be pursued with greater efficacy. 2° By experience; for we see that those who pursue an object with love are carried toward it with greater force than if they were not affected by this passion. Therefore.

The second part is proved. Perfect voluntariness is that which proceeds from an intrinsic principle with knowledge of the end formally as end, that is, with knowledge of the dignity of the end and of the proportion of the means to it. But concupiscence diminishes, nay sometimes takes away this knowledge. Therefore. — The minor is proved. For it diminishes the judgment of reason, nay sometimes destroys it and causes madness, as experience shows. But knowledge of the end formally as end pertains to the judgment of reason. Therefore.

The third part is proved. Free voluntariness is that which proceeds from an intrinsic principle with knowledge of the end under indifference concerning the thing proposed. But antecedent concupiscence diminishes this indifference. Therefore. — The minor is proved: 1° Because it diminishes the judgment of reason, as I was just saying. 2° Because concupiscence causes the intellect to propose to the will, in pursuing the object, all the aspects of the desirable and deficient good, and to attend less to the aspects of the honest good by which the will would be withdrawn from that object. Therefore it diminishes indifference.

Hence you will infer that antecedent concupiscence diminishes sin, insofar as it disturbs the judgment and diminishes liberty, nay sometimes exempts from all sin, when it is so violent that it altogether takes away the judgment of reason and indifference, and renders one mad.