228
In short, if God wishes to share his good with a potential intellectual nature, like ours, then this nature must have the various powers that we find ourselves to have. God is not constrained to make such a nature, but if he wishes to share his good, then this is the nature he wishes to make.1
Part of what it means to have this nature is to have inclinations. A human nature is not some mathematical nature with inclinations patched onto it. A human nature in itself is ordered to an end. The making of the nature is the making of the order to the end. "The nature of anything is a certain inclination (implanted in it by the first mover) ordering it toward its proper end."2 God does not give us inclinations so that we can figure out our end; rather, he gives us an end, which is to give us inclinations, and he gives us a mind by which we can grasp reality, including the reality of our own ends.
God need not jury-rig [improvisar] a special practical knowledge with preset goods. He need only create a power that can grasp the world around it. This power, with no special default settings, becomes aware of the ends in nature. It does not become aware of what is going to be in the future; rather, it becomes aware of what already is present within nature. God need not jury-rig the will to desire certain goods. He need only create the will as a power that seeks the good of the whole person as perceived by reason. When reason grasps the various particular goods, the will by nature desires.
In short, the natural law is not a special setting of our nature; it is the spontaneous outgrowth of our nature. There is nothing "hypothetical" (that is, optional) about the natural law and the ought-statements discovered by our reason. It is part of our nature, inseparable from any special moving force of reason; it arises from what we are. We can no more throw off the moral ought than we can throw off our natures.

…p. 229 The real divide between Aquinas and modern ethical theory is not whether we can derive an ought-statement from an is-statement; it is whether we have a human nature with an impetus at its center, driving onward toward our completion. Barring such a nature, ethics is only a game people play.
Modern man can wish to throw off his nature; nevertheless, the nature remains. It continues to draw us on to a fulfillment beyond ourselves.
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1 See Summa contra Gentiles, II, cap. 28-30; Q. de potentia Dei 3,15
2 41. In Metaphysicorum, lib. 12, lect. 12, n. 2634. "Ipsa natua uniuscuiusque est quaedam inclinatio indita ei a primo movente, ordinans ipsam in debitum finem,"