Philo discusses both the immortality of the written law and the obligation of observing the customs—the unwritten law. Although the Greek world had a concept of unwritten law, Philo’s view is clearly informed by Jewish tradition and by the Pharisaic concept of tradition.

Another most admirable injunction is that nothing should be added or taken away,⁴¹ but all the laws originally ordained should be kept unaltered just as they were. For what actually happens, as we clearly see, is that it is the unjust which is added and the just which is taken away, for the wise legislator has omitted nothing which can give possession of justice whole and complete. Further, he suggests also that the summit of perfection had been reached in each of the other virtues. For each of them is defective in nothing, complete in its self-wrought consummateness, so that if there be any adding or taking away, its whole being is changed and transformed into the opposite condition…

In the same way too, if one adds anything small or great to the queen of virtues—piety—or on the other hand takes something from it, in either case he will change and transform its nature. Addition will beget superstition and subtraction will beget impiety, and so piety too is lost to sight, that sun whose rising and shining is a blessing we may well pray for, because it is the source of the greatest of blessings, since it gives the knowledge of the service of God, which we must hold as lordlier than any lordship, more royal than any sovereignty. Much the same may be said of the other virtues, but as it is habit to avoid lengthy discussions by abridgment, I will content myself with the aforesaid examples, which will sufficiently indicate what is left unsaid.

Another commandment of general value is: “Thou shalt not remove thy neighbor’s landmarks which thy forerunners have set up.”⁴² Now this law, we may consider, applies not merely to allotments and boundaries of land in order to eliminate covetousness but also to the safeguarding of ancient customs. For customs are unwritten laws—the decisions approved by men of old—not inscribed on monuments nor on leaves of paper which the moth destroys, but on the souls of those who are partners in the same citizenship. For children ought to inherit from their parents, besides their property, ancestral customs which they were reared in and have lived with even from the cradle, and not despise them because they have been handed down without written record. Praise cannot be duly given to one who obeys the written laws, since he acts under the admonition of restraint and the fear of punishment. But he who faithfully observes the unwritten deserves commendation, since the virtue which he displays is freely willed.

40. Trans. F. H. Colson, Philo (Loeb Classical Library; Cambridge- Harvard University Press, 1968), vol.
8, 97-103.

41. Deut. 4-2.

42. Deut. 19