But man acts from judgment, because by his apprehensive power he judges that something should be avoided or sought. But because this judgment, in the case of some particular act, is not from a natural instinct, but from some act of comparison in the reason, therefore he acts from free judgment and retains the power of being inclined to various things. For reason in contingent matters may follow opposite courses, as we see in dialectic syllogisms and rhetorical arguments. Now particular operations are contingent, and therefore in such matters the judgment of reason may follow opposite courses, and is not determinate to one. And forasmuch as man is rational is it necessary that man have a free-will.
[ST 1.83.1] The "dialectic syllogisms and rhetorical arguments" part is interesting. Dialectical syllogisms and rhetorical arguments are probable inferences involving judgments that "may follow opposite courses" and are "not determinate to one" because they involve contingent particulars; therefore reason is to this extent itself a free power, and will, as following on reason as rational desire, therefore must have choice appropriate to this. Thus we have free will.
