“The fool doth think he is wise, but the wise man knows himself to be a fool.”
Recognizing the limits of one’s knowledge is presented as a mark of wisdom, while certainty about one’s own wisdom can be evidence of the opposite.
Source details
As You Like It, Act V, Scene 1, Touchstone; 1919 Yale Shakespeare edition edited by Jack R. Crawford, printed page 89 / Wikisource scan leaf 101. Touchstone says, “The fool doth think he is wise, but the wise man knows himself to be a fool,” while verbally outmaneuvering William in a comic exchange over Audrey. The maxim remains fictional comic dialogue rather than a direct Shakespearean self-help proposition.