Socrates (c. 469 BC – 399 BC) was a classical Greek philosopher and is one of the founders of Western philosophy. He is known chiefly through the writings of his students Plato and Xenophon, and the plays of his contemporary Aristophanes. He influenced subsequent philosophers by his approach to logic (named the Socratic Method) and his infamous trial and subsequent execution, as described by Plato.

Socrates on Know Thyself

The only true wisdom is in knowing you know nothing.

“Wisdom begins with wonder,” said Socrates. Through dialogue, he led his audience to passionate inquiry of existence and identity. His speech maintained a humble tone, claiming, to the surprise of his listeners, that he knew nothing. In his mind, one could not know anything without knowing one’s self. Thus, the Seven Sages of Greece, who had inscribed know thyself in the forecourt of the Delphic oracle a few generations before Socrates, had challenged all subsequent philosophers to attain self-knowledge before knowing anything else. Socrates embraced this ancient challenge humbly:

I am not yet able, as the Delphic inscription has it, to know myself; so it seems to me ridiculous, when I do not yet know that, to investigate irrelevant things.
Care first about the greatest perfection of the soul.

“What I want to discover at present,” said Socrates, “is the art which devotes its attention to precision, exactness, and the fullest truth.” His philosophical inquiry was, therefore, dedicated to truth, foremost the truth about himself. By publicly admitting his self-ignorance, he made his audience aware of their own.

Further Reading:
Plato on Know Thyself
Heraclitus on Know Thyself
Thales on Know Thyself