How do proponents of the New Natural Law Theory (NNLT), such as John Finnis & Germain Grisez, define natural law?

Question

I looked in the main books of NNLT, namely Natural Law and Natural Rights by John Finnis and The Way of the Lord Jesus by Germain Grisez, but I did not find a definition of natural law.

Answer 1

Germain Grisez, The Way of the Lord Jesus (vol. 1): Christian Moral Principles, ch. 7 "Natural Law and the Fundamental Principles of Morality", § "Question A: What does ‘natural law’ mean in Catholic teaching?":

1. According to St. Paul, even the Gentiles find the requirements of morality which conscience discerns written in their hearts. Although Gentiles do not have the law divinely revealed to the Jews, they naturally do have this given standard of conduct (see Rom 2:14–16).¹
2. The Church calls these naturally known principles “natural law.” They are natural in the sense that they are not humanly enacted but are objective principles which originate in human nature […] Thus, in speaking of natural law, we are not contrasting “natural” meaning “physical” with “intellectual.” As a matter of fact, natural law does pertain to intellect. Nor is the contrast primarily that of “natural” with “supernatural,” for natural law overlaps divinely revealed law.

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^{1. See C. Spicq, O.P., Théologie Morale du Nouveau Testament, 14th ed., vol. 1 (Paris: J. Gabalda, 1970), 394–406.}