Miguel de Cervantes Saavedra (circa 1547 – 22 April 1616) was a Spanish novelist, poet, and playwright. His magnum opus, Don Quixote, considered the first modern European novel, is a classic of Western literature, and is regarded amongst the best works of fiction ever written. His influence on the Spanish language has been so great that the language is often called la lengua de Cervantes (“the language of Cervantes”)

Cervantes on Self-Knowledge

Thou must keep in view what thou art, striving to know thyself, the most difficult thing to know that the mind can imagine.

This citation is extracted from several paragraphs of advice Don Quixote gives a youth whom he meets and befriends. Much of the irony of Don Quixote stems from his contradictory appearance: while his demeanor suggests that he is mad, his word prove to be full of wisdom. Here, Cervantes repeats the ancient aphorism attributed to Thales, who claimed that the most difficult thing was to know thyself.

If thou knowest thyself, it will follow thou wilt not puff thyself up like the frog that strove to make himself as large as the ox.