The concepts of sub-creation and free will are very closely associated in Tolkien’s mind, and in at least one place he uses them almost interchangeably: “having mentioned Free Will, I might say that in my myth I have used ‘subcreation’ in a special way… Free Will is derivative, and is [therefore] only operative within provided circumstances…” (Letters #153). As the paradigmatic instance of free will, sub-creation becomes for Tolkien something of a model for free action in general. Human praxis, as it were, is a kind of human poesis–human doing a form of human making–inasmuch as every human action seeks to bring about an alternative state of affairs, and therefore to realize an alternative, “secondary world” or reality to the one currently realized. (As Frodo and Sam realize on the stairs of Cirith Ungol, their own heroic quest to destroy the Ring of Sauron and so save Middle-earth is in fact part of an ancient and on-going “tale” that never ends, “[b]ut the people in them come, and go when their part’s ended.”)
The theme of free will, and especially its relationship to divine providence, has received a good deal of attention in the literature on Tolkien, but what I’m presently interested in here (as usual) is the uniquely metaphysical approach Tolkien also takes to this important issue, an approach that, again, leads one back to his Thomistic doctrine of creation. For Tolkien, not only does sub-creative free will dimly mirror the freedom the Creator himself enjoys in the act of creation, but as with its specific application in sub-creation, creaturely free will is likewise wholly dependent for its very existence and exercise upon divine providence. This dependence, however, involves much more than the Creator passively “allowing” or “permitting” his creatures to make their own choices about things (though Tolkien will also speak of the Creator as “accepting” or “permitting” creaturely sub-creating or “Making” when it is used for evil purposes). As Tolkien puts it, free will is not absolute but “derivative,” being “only operative within provided circumstances,” namely, those circumstances in which the Creator himself “should guarantee it” by giving it the “reality of creation.” It is something like this radical sense of causal dependency that the Ainulindalë hints at on its opening page when Ilúvatar invites the Ainur to develop their Music, explaining to them that “since I have kindled you with the Flame Imperishable, ye shall show forth your powers in adorning this theme, each with his own thoughts and devices, if he will” (S 15, emphasis added). For Tolkien, creaturely freedom is not and cannot be threatened by divine providence, for it is the divine Creator who first brings the creaturely free will into being and by whose providence the individual will, its intentions, and its consequent, real-world effects, are continuously and actively kept in being.
 On the relationship between free will and divine providence in Tolkien, see Daniel Timmons’s article “Free Will” and attached bibliography in Drout, ed. J.R.R. Tolkien Encyclopedia.
 See, for example, Letters 190n, 195, and 259.