The profound differences between Tolkien’s Men and Elves may be no less accounted for in hylomorphic (matter-form) terms. What gives Elves their immortality or, more properly speaking, their “serial longevity” (the natural Elvish life being limited to the lifespan of Arda–Morgoth’s Ring 331), is the greater power or strength their souls exercise over their bodies. As a more powerful bodily form or forming principle, in other words, the Elvish soul exercises a greater degree of “command,” “control,” or “mastery,” as Tolkien variously puts it, over its matter, the body (211, 218, 233, 331, and 334). (In another passage Tolkien writes of the Elvish body being “modified by the indwelling fëar” or soul .) The consequence is that the Elvish soul is capable of keeping its body indefinitely alive (provided it is not catastrophically injured), strong, and in good health (427), not to mention “by nature continent and steadfast” (211-13). As Tolkien writes at some length of the Elvish physiognomy:
They were thus capable of far greater and longer physical exertions (in pursuit of some dominant purpose of their minds) without weariness; they were not subject to diseases; they healed rapidly and completely after injuries that would have proved fatal to Men; and they could endure great physical pain for long periods. Their bodies could not, however, survive vital injuries, or violent assaults upon their structure; nor replace missing members (such as a hand hewn off). On the reverse side: the Elves could die, and did die, by their will; as for example because of great grief or bereavement, or because of the frustration of their dominant desires and purposes. This willful death was not regarded as wicked, but it was a fault implying some defect or taint in the fëa, and those who came to Mandos by this means might be refused further incarnate life. (341)
In Tolkien’s Elves, it would seem, we get a fictional and dramatic exaggeration of the Aristotelian-Thomistic doctrine that it is through the formal principle of the soul that the material principle of the body has its being, its life, and its operations.