Lacan on Desire

From Dylan Evans' An Introductory Dictionary of Lacanian Psychoanalysis (London: Routledge, 1996), p. 37:

Need is a purely biological instinct, an appetite which emerges according to the requirements of the organism and which abates completely (even if only temporarily) when satisfied. The human subject, being born in a state of helplessness, is unable to satisfy its own needs, and hence depends on the Other to help it satisfy them. In order to get the Other's help, the infant must express its needs vocally; need must be articulated in demand. The primitive demands of the infant may only be inarticulate screams, but they serve to bring the Other to minister to the infant's needs. However, the presence of the Other soon acquires an importance in itself, an importance that goes beyond the satisfaction of need, since this presence symbolizes the Other's love. Hence demand soon takes on a double function, serving both as an articulation of need and as a demand for love. However, whereas the Other can provide the objects which the subject requires to satisfy his needs, the Other cannot provide that unconditional love which the subject craves. Hence even after the needs which were articulated in demand have been satisfied, the other aspect of demand, the craving for love, remains unsatisfied, and this leftover is desire. 'Desire is neither the appetite for satisfaction, nor the demand for love, but the difference that results from the subtraction of the first from the second' (Ecrits, 287).

Desire is thus the surplus produced by the articulation of need in demand; 'Desire begins to take shape in the margin in which demand becomes separated from need' (Ecrits, 311). Unlike a need, which can be satisfied and which then ceases to motivate the subject until another need arises, desire can never be satisfied; it is constant in its pressure, and eternal. The realisation of desire does not consist in its being 'fulfilled', but in the reproduction of desire as such.