I still think the main disagreement in this thread is that we’re using different conceptions of what “free” means. For most atheists I know, and certainly myself, to say that a choice is free is a modal claim. It means the choice could have been made differently.
The Catholics here seem to be using a very unusual notion of freedom. The main purpose of their version of freedom seems to be to attach responsibility to actions rather than make modal claims about them. Thus they say we are responsible for our “free” choices even when they couldn’t have been made differently.
Here’s a summary on Aquinas’ view on the human will:
"First off, let us treat the will. Generically,
the will is an appetite, that is, a power of the soul by which we are inclined toward something. By means of appetitive powers, we seek and desire things; we strive to unite ourselves (in various ways) with them. They are consequent upon knowledge. “Some inclination follows every form.”( ST, Ia, 80, 1 ) Because knowledge the attainment of a new form in a non-material way, an inclination of the appetite follows upon this knowledge.
So, since there are two kinds of knowledge, sense and intellectual, there are consequently two kinds of appetites.
From sense knowledge, ie. the apprehension of the forms of things in their particularity, sensual appetition follows. In a like manner, from intellectual knowledge, the apprehension of universal forms, intellectual appetition follows. In humans, the intellect is discursive, going from premises to conclusions logically, and so is called rational. Likewise the consequent appetite is rational; it is called the will.
The will then is that power by which we desire the universal, not bound in itself to any manifestation of that universal in particular, real, material things.
The object of both appetites is proportionate to the kind of knowledge appropriate to that appetite.
Now all appetites tend toward goodness in some manner, and the manner of tending is determined by the kind of form it has, that is the kind of knowledge. So, sense appetite tends toward sensible, particular goods, and the will toward universal goodness."
Here’s a more precise definition of freedom:
"Freedom does not apply to the necessities of nature or the general end, for these are not the objects of the will (except perhaps the latter in the next life) but the principles of willing. Freedom does apply to the exercise or non-exercise of the will and to the acts of the will with regard to particular ends as means to the general end. Citing Aristotle in chapter 48 of the second book of the Summa Contra Gentiles, St. Thomas says that the will is the principle of the self motion in humans.
That is, humans are free to exercise their will and to choose particular things.(DV 24.1.)
The will of necessity wills universal goodness as it end and elicits its act by means of reason. It cannot do otherwise while still being a rational appetite. Since this tendency is according to the will, it is voluntary. Any choice of the will for universal goodness, because it is in accord with the principles of volition, is voluntary. Aquinas does not consider this general tendency toward the good as free since it is determined necessarily.(ST Ia, 83, 1 ad 5.)
The will however is an intellectual power, its activity presupposes knowledge.
In considering the voluntary movement toward universal goodness as being necessary and not free, Aquinas is considering a very special case: knowledge of universal goodness. For Aquinas, the Good itself is God, and knowledge of the Good is attainable only in the beatific vision after death. In this situation, we will will voluntarily the Good, but not do it freely.
This point on knowledge is the fundamental foundation for the wills freedom. In the present life, no one object can be considered by our reason as totally good and so does not move the will necessarily to will it in particular. The will must necessarily be moved by the good or apprehended good thing,
no one thing so exhausts goodness that the will is moved to it necessarily.( ST Ia, 105, 4) Many things possess goodness to varying degrees and with various aspects, such that several particular goods may be contrary or, in their various aspects, even contradictory.
Therefore no one thing is sufficient to move the will necessarily. Not even the consideration of the goodness of things is completely good, so that people are free to judge or not judge particular objects. Hence, the will is free both in its exercise and in its specification, ie. choosing one good over the other.
On the other side, in the way the will acts, freedom is also preserved, since every act of the will is particular. The specific act of the will cannot encompass the entirety of universal goodness be cause of its specificity.
So even choosing to act over not acting does not fulfill the will’s necessity toward goodness.( ST Ia-IIae, 13, 6) Also, because no particular means is necessary for the attainment of goodness or happiness, there is freedom. The will might not choose any particular means to its end because it is not bound by the necessity of utility.(DV 24.2.) The will is free to specify its acts after it chooses to act at all."
Aquinas and the Freedom of the Will
So in short, the will is the appetite which is directed towards Goodness, both the universal and the particular.
And no particular thing(or things) is sufficient to move the will by necessity. Therefore when the will chooses a particular thing based upon its goodness(or in the case of sin its perceived goodness), it chooses freely.