The motto γνῶθι σ(ε)αυτόν (“know thyself”) was one of the maxims inscribed on the pediment of the temple of Apollo at Delphi, along with μηδὲν ἄγαν (“nothing in excess”), inviting mankind to exercise moderation in life. It was through these maxims that Apollo’s oracle – one could think of it as one of the “mass-media” of ancient times – invited men to self-investigation, prompting them to discover that the essence of one’s life is not to be searched for in the world outside, but instead within ourselves.
This emphasis on the inner life and on the importance of knowing oneself will become a core element of the philosophy of Socrates, whose Protagoras (343a-b) traces the history of the Delphic inscription back to the Greek Seven Sages:
Such men were Thales of Miletus, Pittacus of Mytilene, Bias of Priene, Solon of our city, Cleobulus of Lindus, Myson of Chen, and, last of the traditional seven, Chilon of Sparta. All these were enthusiasts, lovers and disciples of the Spartan culture; and you can recognize that character in their wisdom by the short, memorable sayings that fell from each of them: they assembled together and dedicated these as the first-fruits of their lore to Apollo in his Delphic temple, inscribing there those maxims which are on every tongue—“Know thyself” and “Nothing overmuch.(μηδὲν ἄγαν)”.(Translation by W. R. M. Lamb)