St. Thomas argues as follows when answering the question whether the practical and speculative intellects are numerically diverse powers:
The practical and speculative intellects are not diverse powers. The reason for this is that . . . that which is accidentally related to the ratio [or account] of the object which bears on some power does not diversify the power. For, it happens to be the case that the thing with color be a man, or large, or small. Whence all things of this sort are apprehended by the selfsame power of sight. However, it happens to be the case in regards to something apprehended by the intellect that it be ordered or not ordered to operation [opus]. Yet this is how the speculative intellect differs from the practical intellect. For the speculative intellect is that which does not order what it apprehends to operation but only to the consideration of the truth, while the practical intellect is that which does order what it apprehends to operation.
[I]ntellectus practicus et speculativus non sunt diversae potentiae. Cuius ratio est quia, ut supra dictum est, id quod accidentaliter se habet ad obiecti rationem quam respicit aliqua potentia, non diversificat potentiam, accidit enim colorato quod sit homo, aut magnum aut parvum; unde omnia huiusmodi eadem visiva potentia apprehenduntur. Accidit autem alicui apprehenso per intellectum, quod ordinetur ad opus, vel non ordinetur. Secundum hoc autem differunt intellectus speculativus et practicus. Nam intellectus speculativus est, qui quod apprehendit, non ordinat ad opus, sed ad solam veritatis considerationem, practicus vero intellectus dicitur, qui hoc quod apprehendit, ordinat ad opus. ~ St. Thomas, Summa Theologiae, Ia, q. 79, a. 11, c.; cf. In Ethic., I, lect. 1, nn. 1–2
Now, in answering the question, St. Thomas requires as a premise that “it happens to be the case in regards to something apprehended by the intellect that it be ordered or not ordered to operation.” That is, it is accidental to the objects of intellectual apprehension that they are in fact ordered to operation or not, and it thus follows that it is accidental to the objects of intellectual apprehension that they are orderable to operation or not. Now, the connatural object of the human intellect is categorical being—ens—and, consequently, the natures of beings. It follows that it is not per se to the natures of beings that they are orderable to operation, namely, the operability in question here, human operation. It is accidental to beings that they can be worked on by us.
Since all that is per accidens is reducible to what is per se, we must point out to what the accidental in this case is accidental: human nature itself. What determines whether or not the nature of a being is operable by us and therefore orderable to operation is the possibility within our nature in regard to the nature of the being (whether substance or accident, including acting and suffering) in question (cf. Ethics, III.3 and VI.2). In and of themselves, natures are not per se operable by us, but only within the technical and moral parameters of possibility and permissibility. Natures—all natures—thus have, to various degrees, a inviolable or sacred character because of their priority over and against ourselves (and this includes our own nature, since material individuals are not their own natures), and attempts to draw the lines of the technical and moral parameters of our operability consequently distinguish theories of ethics. (Perhaps angelic ‘ethics’ must therefore be founded metaphysically on the distinction of essence and existence and not nature and supposit.)
What if the contrary were in fact the case? That is, take this statement contrary to the one in the previous paragraph: “In and of themselves, natures are per se actionable by us.” Here we must make the case sharper—the original claim (in order for St. Thomas’ argument to be valid) must be: “No natures as such are per se actionable by us.” The opposing claims could either be “Some natures as such are per se actionable by us,” or “All natures as such are per se actionable by us.” Either claim, if true, would result in the fact that the practical intellect is a numerically different power that the speculative intellect. On the basis of the particular claim, there would exist by nature some natures in the order of being—much like there now exists the accidental beings of technology—whose to be’s were nothing more than tools or artifacts. The stronger, universal claim would result in the complete duplication of speculative and practical realms in the universe, which would run parallel to each other. There would be speculative truths about things, but these truths would be malleable at will, for their very natures are operable. Which world would come first? Since our very own natures—those of ourselves and our neighbors—would be part of this parallel system of unsystematically malleable truth and fiction, it is unavoidable that the purely creative intellect would come first. Its strictures would be no god and no man.
The consequences of this universal counterfactual, the heart of modern philosophy, are delineated by De Koninck (p. 73 ff). Such consequences result directly from an inability to see the implicit basis of St. Thomas’ argument and its assumption of the primacy of speculative truth.