In short, grace cannot answer to natural need as naturally known; for if it did it would not be grace.
Nor will it do to say that while grace does not answer to natural need, it does answer to the needs of a fallen human nature, as if it were that because of sin, and because we are creatures fallen, grace is demanded because our natures demand to be restored to their innocent condition. At any rate, Thomas does not believe this: in fact, the objection is obvious, grace being, in name and nature, “gratuitous,” it cannot be demanded by anything, except in a purely hypothetical sense: if we are to be saved then only grace can save us...for Thomas the fall is a human predicament which imposes absolutely no obligation on God to do anything about. Fallen as we are, without grace we have no right to anything but to stew in our own juice— in fact, fallen as we are, we hardly even know how far we are fallen; at best we can know that ours is a predicament, our ignorance of its nature being itself an aspect of that self-same fallen condition. For ours is like the condition of the person who is self-deceived : not only is he self-ignorant, he has somehow managed to hide from himself how it is that he is himself the cause of that ignorance and that he has a reason for remaining in it undisturbed. Therefore, for Thomas it takes grace to know that we are in need of grace; and it takes grace for us to know that there is a possible condition to which nature is restored, a condition far beyond the powers of nature even as they were before the Fall. It is in that sense that, for Thomas, nature is “perfected” by grace— not as if, knowing what we want, human beings are by grace given the gift of it, but rather, not knowing what we want, the gift of grace reveals to us the depth and nature of our need, a need that, as heretofore we were, was unknown to us.
Grace, therefore, does not exactly answer to our desire, as if we knew what our desire is. Grace answers to desires that only it can arouse in us, showing us what it is that we really want: grace is pure gift, the gift we could not have known that we wanted until we were given it. For grace does not merely solve the problem of the gap opened up by the Fall, restoring us to where we were before Adam's sin. It goes far beyond and above that, calling us into a friendship which is surplus by an infinite degree to the solution required. We need to come to know this, and to live by the knowledge that everything transacted between ourselves and God— that is, everything to do with the friendship that Jesus offered his followers— is the work of grace, a work that is of its nature supererogatory, being a solution that far exceeds what is needed by the problem it solves.
Thomas Aquinas by Denys Turner