The psychological idea of identity in humans is related to self image, namely a person's view or mental model of him or herself, usually known as their personal identity (see identity crisis and the work of Erik Erikson.)

Sociology recognizes the concept of identity negotiation where a person has multiple identities that relate to different aspects of their life and personality and "negotiates" with society as to the meaning of that identity in their life.

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In philosophy, the self is the idea of a unified being which is the source of an idiosyncratic consciousness. Moreover, this self is the agent responsible for the thoughts and actions of an individual to which they are ascribed. It is a substance, which therefore endures through time; thus, the thoughts and actions at different moments of time may pertain to the same self (See John Locke's theory of consciousness as the basis of personal identity). As the notion of subject, the "self" has been harshly criticized by Nietzsche at the end of the 19th century, on behalf of what Gilles Deleuze would call a "becoming-other".

Many quotations assert that to truly know thyself is the most difficult thing to achieve. Thales of Miletus, when asked what was difficult, answered in a well-known apophthegm: "To Know Thyself" (γνῶθι σεαυτόν), also attributed to Socrates, and inscribed on the Temple of Apollo at Delphi. Lao Zi in his Tao Te Ching says "Knowing others is wisdom | Knowing the self is enlightenment. | Mastering others requires force | Mastering the self requires strength."

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