mirror-stage. [from Laplanche/Pontalis]

According to Jacques Lacan, a phase in the constitution of the human individual located between the ages of six and eighteen months. Though still in a state of powerlessness and motor incoordination, the infant anticipates on an imaginary plane the apprehension and mastery of its bodily unity. This imaginary unification comes about by means of identification with the image of the counterpart as total Gestalt; it is exemplified concretely by the experience in which the child perceives its own reflection in a mirror. The mirror phase is said to constitute the matrix and first outline of what is to become the ego.

The idea of the mirror phase was one of Lacan's earliest contributions, first proposed at the 1936 Marienbad International Congress of Psycho-Analysts.
The concept is grounded upon a number of empirical data:
a. Data taken from child psychology and comparative psychology concerning the infant's behaviour when confronted with its reflection in a mirror. Lacan draws attention to 'the triumphant assumption of the image, with the accompanying jubilant mimicry and the playful complacency with which the specular identification is controlled.'
b. Data derived from animal ethology, which demonstrates how certain results of maturation and biological organisation are attained solely by the visual perception of the counterpart.
According to Lacan, the import of the mirror phase in human development , is attributable to the prematurity of birth, as evidenced by the anatomically incomplete pyramidal system and the motor incoordination of the first months of life.

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I. As far as the structure of the subject is concerned, the mirror phase is said to represent a genetic moment: the setting up of the first roughcast of the ego. What happens is that the infant perceives in the image of its counterpart—or in its own mirror image—a form (Gestalt) in which it anticipates a bodily unity, which it still objectively lacks (whence its 'jubilation'): in other words, it identifies with this image. This primordial experience is basic to the imaginary nature of the ego, which is constituted right from the start as an 'ideal ego' and as the 'root of the secondary identifications' (1b). It is obvious that from this point of view the subject cannot be equated with the ego, since the latter is an imaginary agency in which the subject tends to become alienated.
II. For Lacan, in so far as the intersubjective relationship bears the mark of the mirror phase, it is an imaginary, dual relationship inevitably characterised by an aggressive tension in which the ego is constituted as another and the other as an alter ego (see 'Imaginary').
III. This approach might be compared to Freud's own views on the transition from auto-erotism—which precedes the formation of an ego—to narcissism proper: what Lacan calls the phantasy of the 'body-in-pieces' (le corps morcele) would thus correspond to the former stage, while the mirror stage would correspond to the onset of primary narcissism. There is one important difference, however: Lacan sees the mirror phase as responsible, retroactively, for the emergence of the phantasy of the body-in-pieces. This type of dialectical relation may be observed in the course of psycho-analytic treatment, where anxiety about fragmentation can at times be seen to arise as a consequence of loss of narcissistic identification, and vice versa.