I was told the other day that in one work on free will, Thomas Aquinas suggested that some randomness / non-determinism was a prerequisite for its existence. Does any one know where he expressed this idea? Did he? It doesn't sound like his sort of philosophy.
What you say reminds me of Boethius's Consolation of Philosophy book 5, "Freewill & God's Foreknowledge," where he argues, through the lips of Lady Philosophy, that "nor, indeed, can any creature be rational, unless he be endowed with free will." Boethius discusses this within the context of necessity/determinism and divine foreknowledge, resolving the problem of how God can necessarily foreknow future events but how this foreknowledge does not necessitate such events, thus enabling us to have freewill.
He, and St. Thomas Aquinas later, distinguish two types of necessity: absolute(ontological necessity) and hypothetical("necessitas ex suppositione" or "ex finis"). St. Thomas discusses this in In Physica lib. 2 l. 9 and In Posteriora Analytica lib. 2 l. 8.
Some examples illustrating hypothetical/conditional necessity:
Boethius writes (Consolation of Philosophy p. 236):
if a man be seated, the supposition of his being seated is necessarily true; and, conversely, if the supposition of his being seated is true, because he is really seated, he must necessarily be sitting.
Also, St. Thomas (Contra Gentiles lib. 1 cap. 67 "That God Knows Future Contingent Singulars" n. 10.):
it is necessary that Socrates be seated from the fact that he is seen seated. But this is not absolutely (absolute) necessary or, as some say, with the necessity of the consequent (necessitate consequentis); it is necessary conditionally (sub conditione), or with the necessity of the consequence (necessitate consequentiae).
In other words: Supposing Socrates is sitting, it is conditionally necessary that he sits. But it is absolutely necessary that Socrates sit when he sits.
This is not necessarily true: "If there is an acorn, there will be an oak tree." The converse is necessary, however, since all oak trees come from acorns: "If there is an oak tree, there was an acorn." This is an example of necessitas ex suppositione (suppositional necessity) or ex finis ("necessity from the end" or "from the final result"). Now, does God's knowing this latter statement necessitate that all acorns become oak trees? Certainly not. Acorns are "free" to be eaten by squirrels, to rot, etc.
Thus, St. Thomas et al. view the world as comprised of natures, which may or may not achieve their ends (e.g., becoming an oak tree, or not, in the example above); they do not believe in a mechanistic, absolutely deterministic universe—due, in part, to the problems that poses for the question of free will and the antinomies that follow from denying free will. In this sense, non-determinism could be said to be a prerequisite for free will.
I can't say I've read Aquinas extensively to say anything authoritative, but it doesn't seem like he would say that free-will requires randomness. Consider this passage from Summa Theologica:
Man has free-will: otherwise counsels, exhortations, commands, prohibitions, rewards, and punishments would be in vain. In order to make this evident, we must observe that some things act without judgment; as a stone moves downwards; and in like manner all things which lack knowledge. And some act from judgment, but not a free judgment; as brute animals. For the sheep, seeing the wolf, judges it a thing to be shunned, from a natural and not a free judgment, because it judges, not from reason, but from natural instinct. And the same thing is to be said of any judgment of brute animals. 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.
From this passage, it seems that free-will requires the ability to act rationally, not randomly.
Furthermore, one would need to consider what for Thomas free-will is: it is not the freedom to make arbitrary choices, but it is the freedom to act in accordance with God's will or contrary to it.