That it was necessary for a Law to be given to Man by God

THE acts of irrational creatures, as they belong to the species, are guided by God according to a natural inclination, consequent upon the nature of the species. Therefore, over and above that, there must be given to men something to guide them in their personal acts, and that we call 'law.'

3 and 4. To them is a law fittingly given, who know what they are about, and have the alternative of doing a thing or leaving it undone. But that is proper to the rational creature only. Therefore the rational creature alone is conceptible of law.*

Hence it is said: I will give my law in their hearts (Jer. xxx, 33): I will write for him my manifold laws (Osee viii, 12).

3.113 : That the acts of the Rational Creature are guided by God, not merely to the realisation of the Specific Type, but also to the realisation of the Individual
3.115 : That the main purpose of the Divine Law is to subordinate Man to God