Question:  Suppose you do a search in St. Thomas’ works for the phrase, “de lege naturae,” on the grounds that you want to see, outside the discussion of ST I-II.94.2, and the Treatise on Law in ST, how St. Thomas decides whether or not a precept, perhaps found in positive law, should be assigned to the law of nature?  Then: where are the main places where that sort of discussion is found?

One reason for looking for such texts would be that we would want to examine the actual considerations that St. Thomas uses in deciding whether a precept belongs to the natural law or not.  For example, if the NNL theorists were right, then there would be only one way that he could establish this, namely, by identifying a fundamental good, and a precept based upon it, which either simply was the precept in question, or from which the precept in question could be deduced.

Suprisingly, the answer to that question (at least, according to a simple search) seems to be that there are two main places: (i) the discussion of whether the exclusivity and inseparability of matrimony belong to the law of nature in the Sentences Commentary, and (ii) the discussion of sacrifices and vows under the virtue of religion in ST II-II.85, 88. 

In neither place does St. Thomas proceed as he should, if the NNL theorists were even vaguely right.  But that is not a point I wish to establish here today.   Rather, I want to draw attention to this interesting text which I came upon from the Sentences Commentary, on the inseparability of marrige:

Respondeo dicendum, quod matrimonium ex intentione naturae ordinatur ad educationem prolis non solum per aliquod tempus, sed per totam vitam prolis. Unde de lege naturae est quod parentes filiis thesaurizent, et filii parentum heredes sint; et ideo, cum proles sit commune bonum viri et uxoris, oportet eorum societatem perpetuo permanere indivisam secundum legis naturae dictamen; et sic inseparabilitas matrimonii est de lege naturae

I reply that it should be said that matrimony, from the intention of nature, is ordered to the education of the children, not for a certain time only, but rather for the entire life of the children (which is the reason why, by the law of nature, parents are to lay up wealth for their children, and children should inherit from their parents).  For that reason, since children are a common good of husband and wife, it is necessary according to the dictum of the law of nature that that association remain perpetually undivided. Thus, the inseparability of matrimony is based on the law of nature.—Super Sent., lib. 4 d. 33 q. 2 a. 1 co.

I thought it interesting that St. Thomas argues that the parents’ education of their children continues throughout the adult life of the children, and his appeal to practices of inheritance in this connection is ingenious.  (I am aware that some Catholics wonder why marriage should not be indissoluble only so long as the children are immature.)