J.R.R. Tolkien (or Christopher?) in his notes on “Athrabeth Finrod ah Andreth” (see more here).
Finrod, however, sees now that, as things were, no created thing or being in Arda, or in all Eä, was powerful enough to counteract or heal Evil: that is to subdue Melkor (in his present person, reduced though that was) and the Evil that he had dissipated and sent out from himself into the very structure of the world.
Only Eru himself could do this. Therefore, since it was unthinkable that Eru would abandon the world to the ultimate triumph and domination of Melkor (which could mean its ruin and reduction to chaos), Eru Himself must at some time come to oppose Melkor. But Eru could not enter wholly into the world and its history, which is, however great, only a finite Drama. He must as Author always remain ‘outside’ the Drama, even though that Drama depends on His design and His will for its beginning and continuance, in every detail and moment. Finrod therefore thinks that He will, when He comes have to be both ‘outside’ and inside and so he glimpses the possibility of complexity or of distinctions in the nature of Eru which nonetheless leaves Him ‘The One’.
Since Finrod had already guessed that the redemptive function was originally, specially assigned to Men, he probably proceeded to the expectation that ‘the coming of Eru’, if it took place, would be specially and primarily concerned with Men: that is to an imaginative guess or vision that Eru would come incarnated in human form. This, however, does not appear in the Athrabeth.
We are here dealing with Elvish thought at an early period, when the Eldar were still fully ‘physical’ in bodily form. Much later when the process (already glimpsed by Finrod) called ‘waning’ or ‘fading’ had become more effective, their views of the End of Arda, so far as it affected themselves, must have been modified. But there are few records of any contacts of Elvish and Human thought in such latter days. They eventually became housed, if it can be called that, not in actual visible and tangible hröar, but only in the memory of the fëa of its bodily form and its desire for it and therefore not dependent for mere existence upon the material of Arda.* But they appear to have held, and indeed still to hold, that this desire for the hröa shows that their later (and present) condition is not natural to them, and they remain in estel that Eru will heal it. ‘Not natural’, whether it is due wholly, as they earlier thought, to the weakening of the hröa (derived from the debility introduced by Melkor into the substance of Arda upon which it must feed), or partly to the inevitable working of a dominant fëa upon a material hröa through many ages. (In the latter case ‘natural’ can refer only to an ideal state, in which unmarred matter could for ever endure the indwelling of a perfectly adapted fëa. It cannot refer to the actual design of Eru, since the Themes of the Children were introduced after the arising of the discords of Melkor. The ‘waning’ of the Elvish hröar must therefore be part of the History of Arda as envisaged by Eru, and the mode in which the Elves were to make way for the Dominion of Men. The Elves find their supersession by Men a mystery, and a cause of grief; for they say that Men, at least so largely governed as they are by the evil of Melkor, have less and less love for Arda in itself, and are largely. busy in destroying it in the attempt to dominate it. They still believe that Eru’s healing of all the griefs of Arda will come now by or through Men; but the Elves’ part in the healing or redemption will be chiefly in the restoration of the love of Arda, to which their memory of the Past and understanding of what might have been will contribute. Arda they say will be destroyed by wicked Men (or the wickedness in Men); but healed through the goodness in Men. The wickedness, the domineering lovelessness, the Elves will offset. By the holiness of good men—their direct attachment to Eru, before and above all Eru’s works—the Elves may be delivered from the last of their griefs: sadness; the sadness that must come even from the unselfish love of anything less than Eru.
Desire. The Elves insisted that ‘desires’, especially such fundamental desires as are here dealt with, were to be taken as indications of the true natures of the Incarnates, and of the direction in which their unmarred fulfillment must lie. They distinguished between desire of the fëa (perception that something right or necessary is not present, leading to desire or hope for it); wish, or personal wish (the feeling of the lack of something, the force of which primarily concerns oneself, and which may have little or no reference to the general fitness of things); illusion, the refusal to recognize that things are not as they should be, leading to the delusion that they are as one would desire them to be, when they are not so. (The last might now be called ‘wishful thinking’, legitimately; but this term, the Elves would say, is quite illegitimate when applied to the first. The last can be disproved by reference to facts. The first not so. Unless desirability is held to be always delusory, and the sole basis for the hope of amendment. But desires of the fëa may often be shown to be reasonable by arguments quite unconnected with personal wish. The fact that they accord with ‘desire’, or even with personal wish, does not invalidate them. Actually the Elves believed that the ‘lightening of the heart’ or the ‘stirring of joy’ (to which they often refer), which may accompany the hearing of a proposition or an argument, is not an indication of its falsity but of the recognition by the fëa that it is on the path of truth.)