Thomas Aquinas, Compendium of Theology

This fact shows that such beings have freedom of choice. The intellect does not act or desire without forming a judgment, as lifeless beings do, nor is the judgment of the intellect the product of natural impulse, as in brutes, but results from a true apprehension of the object. For the intellect perceives the end, the means leading to the end, and the bearing of one on the other. Hence the intellect can be the cause of its own judgment, whereby it desires a good and performs an action for the sake of an end. But what is a cause unto itself, is precisely what we call free. Accordingly the intellect desires and acts in virtue of a free judgment, which is the same as having freedom of choice. Therefore the highest substances enjoy freedom of choice.

Furthermore, that is free which is not tied down to any one definite course. But the appetite of an intellectual substance is not under compulsion to pursue any one definite good, for it follows intellectual apprehension, which embraces good universally. Therefore the appetite of an intelligent substance is free, since it tends toward all good in general.