This series is suspended until further notice (a diplomatic way of saying we're tired of it, it has not resulted in any discussion, and there seems to be a complete lack of interest in the subject). Take heart, however. We are replacing this posting with one that will generate even less interest, and which was settled (as far as orthodox Jews, Christians and Muslims are concerned) over eight centuries ago: whether the natural law can be discerned by reason alone, or whether we need religious revelation to discern what is good and be virtuous people. As far as Thomas Aquinas was concerned, it was reason alone:
Aquinas divides law into four kinds: eternal, natural, human, and Divine.
All humans, if virtuous, can have access to the natural law even without supernatural infusion or revelation. For example, consider what Aquinas says as he fields the following objection to the claim that there is only one Divine Law: "Further, the Divine law seems to be more akin to the eternal law, which is one, than the natural law, according as the revelation of grace is of a higher order than natural knowledge. Therefore much more is the Divine law but one." (ST I-II q. 91 a. 5, obj. 3.). The relevant difference between the eternal law and the natural law to which Aquinas appeals is that the knowledge of natural law is natural — that is, knowledge of the natural law does not depend on "the revelation of grace." This is confirmed in Aquinas's response to the objection, where he writes, "The natural law directs man by way of certain general precepts, common to both the perfect and the imperfect: wherefore it is one and the same for all." (ST` I-II q. 91 a. 5, ad 3.)
Natural law is related to the eternal law, which ultimately consists in God's governance of the universe "by Divine Providence" (ST I-II q. 91 a. 1, resp; cf. ST I q. 22 a. 1-2.) Specifically, natural law just is a "participation of the eternal law." (cf., ST I-II q. 91 a. 2, resp.) That is, the principles of the natural law are a certain kind of manifestation of the eternal law.
Granted, Aquinas claims that the eternal law itself is promulgated in part through Christ and the words of scripture, both matters of revelation; he says, "Promulgation is made by word of mouth or in writing; and in both ways the eternal law is promulgated: because both the Divine Word [i.e., Christ, the Word made flesh] and the writing of the Book of Life are eternal." (ST I-II q. 91 a. 1, ad 2.)
Note, however, what Aquinas says about the promulgation of natural law: "The natural law is promulgated by the very fact that God instilled it into man's mind so as to be known by him naturally." (ST I-II q. 90 a. 4 ad 1.) While the natural law is "an imprint on us of the Divine light," (ST I-II q. 91 a. 2, resp.), this imprint need not be made on us via revelation.
And again: "Now all men know the truth to a certain extent, at least as to the common principles of the natural law: and as to the others, they partake of the knowledge of truth, some more, some less; and in this respect are more or less cognizant of the eternal law." (ST I-II q. 93 a. 2, resp.) It would appear that "all men" is not limited to those who have received revelation.
Consider also the precepts of the natural law; in addition to the general precept that good is to be sought and evil avoided, Aquinas includes the following: "whatever is a means of preserving human life, and of warding off its obstacles," norms regarding "sexual intercourse, education of offspring and so forth," as well as precepts "to shun ignorance, to avoid offending those among whom one has to live, and other such things regarding the [natural inclination to know the truth about God, and to live in society]." (ST I-II q. 94 a. 2, resp.) None of these general principles appear to require revelation.
Further, Aquinas writes, "By the natural law, the eternal law is participated proportionately to the capacity of human nature." (ST I-II q. 91 a. 4, ad 1.) Here, Aquinas compares the natural law to the Divine law; the latter is concerned with truths of revelation, while the former is not - at least, not necessarily. The law of grace applies to the latter, but not necessarily to the former. Pelagian worries can be put to rest insofar as we make the qualification that the bonum honestum towards which the natural law points us is that of the earthly city, not the heavenly city. So, while human virtue won't get to what is unqualifiedly good (i.e., God Himself) without grace, it can aspire to what is good qua earth-bound human.
G. Joyce, writing in The Catholic Encyclopedia, indicates that there is a relationship between the truths of natural law and revelation. He notes, however, that revelation just has a particular role to play in the appropriation of natural law — it is not a sine qua non: "Though his intellectual faculties are not radically vitiated, yet his grasp of truth is weakened; his recognition of the moral law is constantly clouded by doubts and questionings. Revelation gives to his mind the certainty he had lost, and so far repairs the evils consequent on the catastrophe which had befallen him." (G. Joyce, "Revelation," The Catholic Encyclopedia (New Advent), "Revelation.") This is echoed in the Catechism of the Catholic Church: "The precepts of natural law are not perceived by everyone clearly and immediately. In the present situation sinful man needs grace and revelation so moral and religious truths may be known 'by everyone with facility, with firm certainty and with no admixture of error.'[cf., Pius XII, Humani generis] The natural law provides revealed law and grace with a foundation prepared by God and in accordance with the work of the Spirit." (§1960). Again, revelation is not needed to see or know the precepts of natural law; rather, revelation helps to remove doubts about what one already knows and helps one apply it readily.
Later, Joyce asserts that thinking revelation is necessary is, in fact, contrary to Catholic theology: "Luther indeed asserted that man's intellect had become hopelessly obscured by original sin, so that even natural truth was beyond his reach. And the Traditionalists of the nineteenth century (Bautain, Bonnetty, etc.) also fell into error, teaching that man was incapable of arriving at moral and religious truth apart from Revelation." (Joyce)