When, in the Silmarillion, the herald of Manwë reports to him the bold and brazen words of Fëanor, we are told that
Manwë wept and bowed his head. But at that last word of Fëanor: that at the least the Noldor should do deeds to live in song for ever, he raised his head, as one that hears a voice far off, and he said: ‘So shall it be! Dear-bought those songs shall be accounted, and yet shall be well-bought. For the price could be no other. Thus even as Eru spoke to us shall beauty not before conceived be brought into Eä, and evil yet be good to have been.’ (“Of the Sun and Moon and the Hiding of Valinor”)
St. Thomas, however, would seem to prefer not put things in quite this way. In his article on “whether God wills evils” (ST I.19.9), the first objection he entertains reads as follows:
It seems that God wills evils. For every good that exists, God wills. But it is a good that evil should exist. For Augustine says (Enchiridion 95): “Although evil in so far as it is evil is not a good, yet it is good that not only good things should exist, but also evil things.” Therefore God wills evil things.
To this objection Aquinas replies thus:
Some have said that although God does not will evil, yet He wills that evil should be or be done, because, although evil is not a good, yet it is good that evil should be or be done. This they said because things evil in themselves are ordered to some good end; and this order they thought was expressed in the words “that evil should be or be done.” This, however, is not correct; since evil is not of itself ordered to good, but accidentally. For it is beside the intention of the sinner, that any good should follow from his sin; as it was beside the intention of tyrants that the patience of the martyrs should shine forth from all their persecutions. It cannot therefore be said that such an ordering to good is implied in the statement that it is a good thing that evil should be or be done, since nothing is judged of by that which appertains to it accidentally, but by that which belongs to it essentially.
As Aquinas would see it, accordingly, while it is true that not only good, but a unique form of good that otherwise would not have been possible, is brought about as a consequence of Fëanor’s rebellion, it does not follow from this, as Manwë implies, that it was therefore good for Fëanor’s “evil to have been” (indeed, for Aquinas, as for Tolkien generally, since evil has no being of itself but is a privation of being, it makes no sense to speak, literally, of evil “having been”). Manwë’s error, in other words, might be seen to involve the fallacy of division, of assuming, that is, that what is true of the whole (in this case, the goodness of Fëanor’s-evil-leading-to-good) must therefore also be true of its parts (the goodness of Fëanor’s-evil).