Summarizing a line of argument from Aquinas, Robert Spaemann (Essays in Anthropology) notes that Thomas acknowledges that “By nature animals are sufficiently equipped to attain their own end” (15). The question is: Are human beings? And if the end of human beings is union with God, how can we not be equipped to attain that? If we say that, however, aren't we slipping into Pelagianism?
Spaemann quotes Thomas: “Just as nature does not fail man in necessaries, although it has not provided him with weapons and clothing, as it provided other animals, because it gave him reason and hands, with which he is able to get these things for himself; so neither did it fail man in things necessary, although it gave him not the wherewithal to attain Happiness: since this is could not do. But it did give him free-will, with which he can turn to God, that He may make him happy” (quoted 15 from ST I-II, q, 5, a. 5).
Spaemann sees an analogy with Aristotle: “just as for Aristotle to isolate a self-sufficient individual ‘nature' (and its capacities) is to abstract from the social nature of human beings, a nature always including friendship, for Thomas to isolate a ‘pure nature' is to abstract form our religious nature, a nature leading to friendship with God. Human nature's self-transcendence is considered analogous to the way that, through the use of his hands and his mind, the human being overcomes the state of deficiency in which he finds himself as a natural being. . . . The key here is that nature draws out from mankind something more than nature, something ‘more noble' (nobilior), as Thomas has it. A human being is not this surplus; a human being is that creature in which nature transcends itself toward that surplus.”
Spaemann quotes from Pascal: “L'homme transcend infiniment l'homme,” but adds that for Thomas “this self-transcendence is rooted in the teleological make-up of nature itself.” Human beings recover nature only in transcending it: “Only in human beings does what nature really is intrinsically manifest itself. . . . Because only in him does nature's purposive structure become free of ambiguity and appear as both free will and free recognition of a foundation and end he did not posit himself” (15-16)