12 But it may be objected, that the Gentile allows nothing of this sort. Come then, let us discuss this point, and as we have done with respect to the creation, having carried on the warfare against these objectors not only by the help of the Scriptures, but of reason, so also let us now do with respect to conscience. For Paul too, when he was engaged in controversy with such persons, entered upon this head. What then is it that they urge? They say, that there is no self-evident law seated in our consciences; and that God has not implanted this in our nature.
But if so, whence is it, I ask, that legislators have written those laws which are among them concerning marriages, concerning murders, concerning wills, concerning trusts, concerning abstinence from encroachments on one another, and a thousand other things. For the men now living may perchance have learned them from their elders; and they from those who were before them, and these again from those beyond? But from whom did those learn who were the originators and first enactors of laws among them?
Is it not evident that it was from conscience? For they cannot say, that they held communication with Moses; or that they heard the prophets. How could it be so when they were Gentiles? But it is evident that from the very law which God placed in man when He formed him from the beginning, laws were laid down, and arts discovered, and all other things. For the arts too were thus established, their originators having come to the knowledge of them in a self-taught manner.
Source: Homilies on the Statues (New Advent)