Hallucinations are usually defined in psychiatry as "false perceptions," that is, perceptions which arise "in the absence of an appropriate external stimulus."
Lacan finds such definitions inadequate, since they ignore the dimension of meaning and signification.
Hallucinations are a typical phenomenon of psychosis, and are usually auditory (hearing voices), but may also be visual, somatic, tactile, olfactory, or gustatory.
Lacan argues that psychotic hallucinations are a consequence of the operation of foreclosure.
Foreclosure refers to the absence of the Name-of-the-Father from the symbolic universe of the psychotic subject.
A hallucination is the return of this foreclosed signifier in the dimension of the real; "that which has not emerged into the light fo the symbolic appears in the real."
THis is not to be confused with projection, which Lacan regards as a mechanism proper to neurosis rather than psychosis.
In this distinction, Lacan follows Freud's analysis of Schreber's hallucinations.
"It was incorrect to say that the perception which suppressed internally is projected outwards; the truth is rather, as we now see, that what was abolished internally returns from without.
While hallucinations are most commonly associated with psychosis, there is another sense in which they play an important part in the structure of desire in all subjects.
Freud argues that "[t]he first wishing seems to have been a hallucinatory cathecting of the memory of satisfaction."