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The Ancient Greek aphorism "know thyself" (Greek: γνῶθι σεαυτόν, transliterated: gnōthi seauton; also ... σαυτόν … sauton with the ε contracted), is one of the Delphic maxims and was inscribed in the pronaos (forecourt) of the Temple of Apollo at Delphi according to the Greek periegetic (travelogue) writer Pausanias (10.24.1).
The maxim, or aphorism, "know thyself" has had a variety of meanings attributed to it in literature. The Suda, a 10th-century encyclopedia of Greek knowledge, says: "the proverb is applied to those whose boasts exceed what they are", and that "know thyself" is a warning to pay no attention to the opinion of the multitude.
The aphorism has been attributed to at least the following ancient Greek sages:
Diogenes Laërtius attributes it to Thales (Lives I.40), but also notes that Antisthenes in his Successions of Philosophers attributes it to Phemonoe, a mythical Greek poetess, though admitting that it was appropriated by Chilon. In a discussion of moderation and self-awareness, the Roman poet Juvenal quotes the phrase in Greek and states that the precept descended e caelo (from heaven) (Satires 11.27). The 10th-century Byzantine encyclopedia the Suda, recognized Chilon and Thales as the sources of the maxim "Know Thyself."
The authenticity of all such attributions has been doubted; according to one pair of modern scholars, "The actual authorship of the three maxims set up on the Delphian temple may be left uncertain. Most likely they were popular proverbs, which tended later to be attributed to particular sages."
Plato employs the maxim 'Know Thyself' extensively by having the character of Socrates use it to motivate his dialogues. Plato makes it clear that Socrates is referring to a long-established wisdom. Benjamin Jowett's index to his translation of the Dialogues of Plato lists six dialogues which discuss or explore the saying of Delphi: 'know thyself.' These dialogues (and the Stephanus numbers indexing the pages where these discussions begin) are Charmides (164D), Protagoras (343B), Phaedrus (229E), Philebus (48C), Laws (II.923A), I Alcibiades (124A, 129A, 132C).
In Plato's Charmides, Critias refers to the maxim consistently with the view expressed in the Suda, with Critias saying, "for they imagined that 'Know Thyself!' was a piece of advice which the god gave and not his salutation of the worshippers at their first coming in." In modern words Critias gives his opinion that 'Know Thyself!' was an admonition to those entering the sacred temple to remember or know their place and Critias says, " 'know thyself!' and 'be temperate!' are the same. Notice that when the words of Critias are written, 'thyself' and 'temperate' are punctuated with exclamation marks in the English translations, as if they were commands. In the balance of the Charmides, Plato has Socrates lead a longer inquiry as to how we may gain knowledge of ourselves.
In Plato's Phaedrus, Socrates uses the maxim 'know thyself' as his explanation to Phaedrus for why he has no time for mythology or other far flung topics. Socrates says, "But I have no leisure for them at all; and the reason, my friend, is this: 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."
In Plato's Protagoras, Socrates lauds the authors of pithy and concise sayings delivered precisely at the right moment and says that Lacedaemon, or Sparta, educates its people to that end. Socrates lists the Seven Sages as Thales, Pittacus, Bias, Solon, Cleobulus, Myson, and Chilon, who he says are gifted in that Lacedaemonian art of concise words "twisted together, like a bowstring, where a slight effort gives great force." Socrates says examples of them are, "the far-famed inscriptions, which are in all men's mouths,--'Know thyself,' and 'Nothing too much'.". Having lauded the maxims, Socrates then spends a great deal of time getting to the bottom of what one of them means, the saying of Pittacus, 'Hard is it to be good.' The irony here is that although the sayings of Delphi bear 'great force,' it is not clear how to live life in accordance with their meanings. Although, the concise and broad nature of the sayings suggests the active partaking in the usage and personal discovery of each maxim; as if the intended nature of the saying lay not in the words but the self-reflection and self-referencing of the person thereof.
In Plato's Philebus dialogue, Socrates refers back to the same usage of 'know thyself' from Phaedrus to build an example of the ridiculous for Protarchus. Socrates says, as he did in Phaedrus, that people make themselves appear ridiculous when they are trying to know obscure things before they know themselves. Plato also alluded to the fact that understanding 'thyself,' would have a greater yielded factor of understanding the nature of a human being. Syllogistically, understanding oneself would enable thyself to have an understanding of others as a result.
Aeschylus uses "know thyself" in its original Greek mythology form in Prometheus Bound. In this play, Prometheus rails against being bound to a cliffside by Zeus, and is cautioned by Ocean that Prometheus should "know thyself". In this context, Ocean telling Prometheus that he should know better then to speak ill of the one who decides his fate, and should know his place.
Aristophanes is the third contemporary of Socrates whose descriptions of Socrates and his teaching remain extant. In The Clouds, Aristophanes makes a mockery of philosophers in general and Socrates in particular. The words γνώσει δὲ σαυτὸν (and know yourself) are used in The Clouds by a father deriding his son for his lack of learning, "And you will know yourself, how ignorant and stupid you are."
From 1539 onwards the phrase nosce te ipsum and its Latin variants were often used in the anonymous texts written for anatomical fugitive sheets printed in Venice as well as for later anatomical atlases printed throughout Europe. The 1530s fugitive sheets are the first instances in which the phrase was applied to knowledge of the human body attained through dissection.
In 1651, Thomas Hobbes used the term nosce teipsum which he translated as 'read thyself' in his famous work, The Leviathan. He was responding to a popular philosophy at the time that you can learn more by studying others than you can from reading books. He asserts that one learns more by studying oneself: particularly the feelings that influence our thoughts and motivate our actions. As Hobbes states, "but to teach us that for the similitude of the thoughts and passions of one man, to the thoughts and passions of another, whosoever looketh into himself and considereth what he doth when he does think, opine, reason, hope, fear, etc., and upon what grounds; he shall thereby read and know what are the thoughts and passions of all other men upon the like occasions."
In 1750 Benjamin Franklin, in his Poor Richard's Almanack, observed the great difficulty of knowing one's self, with: "There are three Things extremely hard, Steel, a Diamond, and to know one's self." 
In 1831, Ralph Waldo Emerson wrote a poem entitled "Γνώθι Σεαυτόν", or Gnothi Seauton ('Know Thyself'), on the theme of 'God in thee.' The poem was an anthem to Emerson's belief that to 'know thyself' meant knowing the God which Emerson felt existed within each person.
In 1832, Samuel T. Coleridge wrote a poem entitled "Self Knowledge" in which the text centers on the Delphic maxim 'Know Thyself' beginning, 'Gnôthi seauton!--and is this the prime And heaven-sprung adage of the olden time!--' and ending with 'Ignore thyself, and strive to know thy God!' Coleridge's text references JUVENAL, xi. 27.
In 1902, Hugo von Hofmannsthal has his 16th-century alter ego in his letter to Francis Bacon mention a book he intended to call "Nosce te ipsum".