Aquinas on Law and Gospel

There is a twofold element in the Law of the Gospel. There is the chief element, namely, the grace of the Holy Spirit bestowed inwardly. And as to this, the New Law justifies. Hence Augustine says "There (that is, in the Old Testament) the Law was set forth in an outward fashion, that the ungodly might be afraid; here (in the New Testament) it is given in an inward manner, that they might be justified." The other element of the Evangelical Law is secondary; namely, the teachings of faith, and those commandments which direct human affections and human actions. And as to this, the New Law does not justify. Hence the Apostle says: "The letter killeth, but the spirit quickeneth" (2 Cor 3.6), and Augustine explains this by saying that the letter denotes any writing that is external to man, even that of the moral precepts such as are contained in the Gospel. Therefore the letter, even of the Gospel, would kill, unless there were the inward presence of the healing grace of faith.

- Thomas Aquinas, Summa Theologica, I-II.106.2
Translated by Fathers of the English Dominican Province (New York: 1946)

Many associate Aquinas with natural law ethics, in which humanity is to do that which conforms to the law of nature and which can be known naturally. However, notice that here Aquinas 'assumes that an adequate theological ethics could not be limited to or based on an analogy with law.'*

I'd love to hear what people think of this quote. Do you agree? Is this what Paul is getting at in 2 Corinthians 3 when he speaks about the letter and the S/spirit?
*Stanley Hauerwas, "On Keeping Theological Ethics Theological" in The Hauerwas Reader, eds. John Berkman and Michael Cartwright (Durham: 2001), 71. The earlier Kant quote was also cited in this essay. Please don't get the impression that my recent enforced convalesence is being hugely productive in terms of reading. This is the final essay we're reading for the reading group I mentioned a while ago. We'll try to decide this morning what to read next.