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This may be proved in three ways. The first…
More on the intellect, the good, and what precisely free will means.
Chapter 48 The intellectual substance are of free-will in acting (alternate translation)
1 FROM this it is clear that the aforesaid substances are of free-will in acting.
2 That they act by judgment is clear, since through their intellective knowledge they judge of things to be done. And they must needs have freedom if, as proved, they have dominion over their action. Therefore the aforesaid substances are of free-will in acting.
Notes Which is plain, since you yourself used your intellective knowledge to judge that it would be well to read this article.
3 Again. The free is that which is its own cause. Wherefore that which is not the cause of its own acting is not free in acting. Now whatever things are not moved, nor act except they be moved by others, are not a cause of their own acting. Therefore self-movers alone have liberty in acting. These alone act by judgment: because the self-mover is divided into mover and moved; and the mover is the appetite moved by intellect, imagination, or sense, to which faculties judgment belongs. Of these then those alone judge freely which in judging move themselves. Now, no judging power moves itself to judge unless it reflect on its own action: for if it moves itself to judge it must needs know its own judgment: and this belongs to the intellect alone. Hence irrational animals have, in a sense, free movement or action, but not free judgment: whereas inanimate beings, which are moved only by others, have not even free action or movement; while intellectual beings have freedom not only of action, but also of judgment, and this is to have free-will.
Notes The italics in the translation for “The free is that which is its own cause” are original and important, especially considering arguments against free will are causative (molecules, atoms, and whatnot act by strict unbreakable “laws” so free will cannot occur). Now “no judging power moves itself to judge unless it reflect on its own action…and this belongs to the intellect alone.” Only the intellect can apprehend. Apprehension is greater than sensation or imagination. Apprehension is what you yourself are doing now by understanding these sentences.
4 Further. The apprehended form is a moving principle according as it is apprehended under the aspect of good or fittingness: because the external action in self-movers comes from the judgment whereby it is judged that something is good or fitting through the aforesaid form. Accordingly, if he who judges moves himself to judge, he must needs, by some higher form, move himself to judge. And this form can be no other than the idea itself of good or fittingness, whereby one judges of any determinate good or fitting thing. Wherefore those alone move themselves to judge who apprehend the common notion of goodness or fittingness. And these are intellectual beings alone. Therefore intellectual beings alone move themselves not only to act, but also to judge. Therefore they alone are free in judging, and this is to have free-will.
Notes We move towards what we think is good in the moment, even though, Lord help us, we often realize some movements were better not made. This says our momentary notions of the good are not accurate apprehensions of the good.
5 Moreover. Movement and action do not follow from a universal concept save through the medium of a particular apprehension: because movement and action are about particular things. Now the intellect is naturally apprehensive of universals. Wherefore, in order that movement and action of any kind follow from the apprehension of the intellect, it is necessary for the universal concept of the intellect to be applied to particulars. But the universal contains many particulars potentially. Hence application of the intellectual concept may be made to many and divers things. Consequently the judgment of the intellect about matters of action is not determined to one thing only. Therefore all intellectual beings have free-will.
Notes And we know “the intellect is naturally apprehensive of universals” because you yourself understand universals. Even claiming not to understand universals would be to claim apprehension of a universals (that there aren’t any), which is self-contradictory. The rest follows easily from choosing among potentialities.
6 Further. Certain things lack liberty of judgment, either because they have no judgment at all, as plants and stones; or because they have a judgment determined by nature to one thing, as irrational animals, for the sheep by its natural estimate judges the wolf to be harmful to it, and as a result of this judgment flies from the wolf; and the same applies to others.
Whatever beings therefore have a judgment that is not determined to one thing by nature, must needs have free-will. Now such are all intellectual beings. For the intellect apprehends not only this or that good, but good itself in general. Wherefore, since the intellect moves the will by the form apprehended; and since in all things mover and moved must needs be mutually proportionate; the will of an intellectual substance will not be determined by nature otherwise than to the good in general. Hence, whatever be offered to it under the aspect of good, it is possible for the will to be inclined thereto, since there is no natural determination to the contrary to prevent it. Therefore in all intellectual beings the will’s act resulting from the judgments of the intellect is free: and this is to have free-will which is defined as the free judgment of reason.
Notes And there you go. Free will is the apprehending and choosing what seems good, the rest following by physics and chemistry, of you like. And this reinforces the ancient notion that the most important thing we can do is know what the good is.