 If an action agrees with the rational and social aspects of human nature, it is permissible; if it doesn’t, it is impermissible (cf. I.1.12.1). That is to say, the source of the natural law is the (in)compatibility of actions with our essences as rational and social beings. For discussion of the etiamsi daremus passage, see St. Leger (1962) and Todescan (2003).