I am currently defending Aguinas’s positon on human law as a derivation of natural law. He describes it in two fashions in his treatise on law. First he describes it as in “sciences, where demonstrated conclusions are drawn from principles, while the second mode is likened to that whereby, in the arts, general forms are particularized as to details.” I understand the first part, but im very confused as to his meaning of the second part involving the derivation in the arts.
For Aquinas, there are two sorts of mental disposition, or virtues, of the intellect: speculative and practical. The speculative dispositions are the theoretical sciences and are pursued for their own sakes. The derivation of natural law as conclusions drawn from principles in the first mode is as a speculative science. Practical virtues are not pursued for their own sake, but for the sake of some further end beyond themselves. They are of two kinds, practical sciences which are pursued for the sake of some human action (things which are done), and the arts, which are pursued for the sake of the things made. Thus, the disposition for ethical reasoning (prudence) exists for the sake of acting well, while the mental virtue of architecture exists for the sake of building houses (or whatever).
The derivation of the natural law insofar as it leads to action belongs to the practical virtue of prudence, and is analogous to the application of an art (as mental disposition) to the actual creation of the artistic work. Both works of art and moral actions are particular and require an ability to apply the general universal principles to the contingency of particular cases. This is the second mode that Thomas talks about, where general forms are particularized as to details. It is important to note, however, that while there are these two divisions of intellectual virtues as speculative and practical, there is only one intellect, and the speculative knowledge of the sciences is employed in acting according to prudence. On the other hand, prudence is necessary as a separate virtue which grows out of experience in applying general moral norms to particular cases since truly speculative knowledge will always remain at a certain level of abstraction, and cannot in principle take the particularities of concrete situations into account in its speculative reasoning.