"Know thyself" (Greek: Γνῶθι σαυτόν, gnōthi sauton)[a] is a philosophical maxim which was inscribed upon the Temple of Apollo in the ancient Greek precinct of Delphi. The best-known of the Delphic maxims, it has been quoted and analyzed by numerous authors throughout history, and has been applied in many ways. Although traditionally attributed to the Seven Sages of Greece, or to the god Apollo himself, the inscription likely had its origin in a popular proverb.
Ion of Chios makes the earliest explicit allusion to the maxim in a fragment dating to the 5th century BC, though the philosopher Heraclitus, active towards the end of the previous century, may also have made reference to the maxim in his works. The principal meaning of the phrase in its original application was "know your limits" – either in the sense of knowing the extent of one's abilities, knowing one's place in the world, or knowing oneself to be mortal. In the 4th century BC, however, the maxim was drastically re-interpreted by Plato, who understood it to mean, broadly speaking, "know your soul".
In later writings on the subject, one common theme was that one could acquire knowledge of the self by studying the universe, or knowledge of the universe by studying the self. This was often explained in terms of the microcosm–macrocosm analogy, the idea that a human being is structurally similar to the cosmos. Another theme, which can be traced back to the Platonic Alcibiades I, is that one can only know oneself by observing other people.
Christian, Jewish and Islamic authors found various scriptural equivalents for the maxim, allowing them to discuss the topic of self-knowledge without reference to the pagan inscription. By the time of the Protestant Reformation, Christian theologians generally understood the maxim to enjoin, firstly, knowledge of the soul's origin in God, and secondly, knowledge of the sinfulness of human nature. In secular writings of the period, several new meanings emerged; among them, that "know thyself" was a command to study the physical properties of the human body.
During the 19th and 20th centuries, the maxim acquired several new associations. It was frequently quoted in German philosophy and literature, by authors such as Kant, Hegel and Goethe; it was cited as an analogue of "tat tvam asi" ("that thou art"), one of the "Great Sayings" of Hinduism; and it took on an important role in the developing discipline of psychoanalysis, where it was interpreted as an injunction to understand the unconscious mind.
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