Philo Judaeus De Abrahamo 5

[5] for these men have been living and rational laws; and the lawgiver has magnified them for two reasons; first, because he was desirous to show that the injunctions which are thus given are not inconsistent with nature; and, secondly, that he might prove that it is not very difficult or laborious for those who wish to live according to the laws established in these books, since the earliest men easily and spontaneously obeyed the unwritten principle of legislation before any one of the particular laws were written down at all. So that a man may very properly say, that the written laws are nothing more than a memorial of the life of the ancients, tracing back in an antiquarian spirit, the actions and reasonings which they adopted;