In clear terms, what is the "mirror stage" as defined by Jaques Lacan's theory of identity?
Jaques Lacan's theory of identity development includes the mirror stage (also mirror phase) between six months of age and eighteen months of age at which time, according to Lacan's theory, an infant first recognizes its image in a mirror or its own image as represented by mother and people. According to the theory, the infant recognizes the mirror image and people as representing a distinct "other" that the infant's previous self-image, that of various parts having no continuity, falls far short of and is therefore inferior to: the mirror image becomes an "imago" to which the individual aspires, usually with no success, all through life.
Lacan holds the theory that the mirror image, the imago, or "ideal ego," originates within the infant feelings of narcissism and love and a desire to emulate the imago. At the same time, according to theory, the imago also inspires envy and dislike that result in confusion and tension--the beginning, according to Lacan, of neuroses. As Leader and Groves explain it (Introducing Lacan, 2000), the infant's conflicting reaction to the imago cements a trauma based on her/is perceived imperfection that results in self-loathing and leads to the desire to become the imago, the ideal ego, which becomes a life long quest. Lacan theorizes that this desire for a connected whole--instead of disparate parts of a self-perceived physical form--and for individual perfection undergirds the tension resulting from the identity (ideal-ego) versus non-identity (self-perception) dichotomy.
Lacan's theory holds that as the infant moves through the "imaginary order" (based on perceptions of a mirror image) of this mirror stage, sh/he builds self-image by oscillating between the imago of these "alien" images and the equally misrepresented self-perception of fragmented body parts. The theorized ultimate result of the mirror stage is that adults have inferior self-images of themselves as integrated and whole individuals. Since the real "me" never can match the imago, the ideal ego of the mirror image, adults may theoretically have either narcissistic fascination or discomfort with their self-image, with their identity, because "I never match my mirror image."