There is a written and an unwritten law. The one by which we regulate our constitutions… — Diogenes Laertius

There is a written and an unwritten law. The one by which we regulate our constitutions in our cities is the written law; that which arises from custom is the unwritten law.

In the original Greek

Νόμου διαιρέσεις δύο: ὁ μὲν γὰρ αὐτοῦ γεγραμμένος, ὁ δὲ ἄγραφος. ᾧ μὲν ἐν ταῖς πόλεσι πολιτευόμεθα, γεγραμμένος ἐστίν. ὁ δὲ κατὰ ἔθη γινόμενος οὗτος ἄγραφος καλεῖται:

The Lives and Opinions of Eminent Philosophers · 240
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Provenance
Current citation
Plato. Quoted in The Lives of Eminent Philosophers, written by Diogenes Laërtius, translated by Pamela Mensch, edited by James Miller. Oxford University Press, 2018, bk. 3, sect. 86.
Original citation
Plato. Quoted in Lives and Opinions of Eminent Philosophers, written by Diogenes Laertius. c. 240 AD, bk. 3, sect. 86.