[11] If they do not sink below a certain level. Since no fea can be annihilated, reduced to zero or not-existing, it is no[t] clear what is meant. Thus Sauron was /said/ to have fallen below the point of ever recovering, though he had previously recovered. What is probably meant is that a 'wicked' spirit becomes fixed in a certain desire or ambition, and if it cannot repent then this desire becomes virtually its whole being. But the desire may be wholly beyond the weakness it has fallen to, and it will then be unable to withdraw its attention from the unobtainable desire, even to attend to itself. It will then remain for ever in impotent desire or memory of desire.
Tolkien, J. R. R. The History of Middle-Earth X: Morgoth’s Ring. Ed. Christopher Tolkien. (London: HarperCollins, 2002.) 407-8. (Myths Transformed, Text VII “Notes on motives in the Silmarillion," (iii), Notes) [This is a note to this passage.]