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Lacan bases this distinction on the fact that in order to satisfy his needs the infant must articulate them in language; in other words, the infant must articulate his needs in a "demand". However, in doing so, something else is introduced which causes a split between need and demand; this is the fact that every demand is not only an articulation of need but also an (unconditional) demand for love. Now, although the other to whom the demand is addressed (in the first instance, the mother) can and may supply the object which satisfies the infant's need, she is never in a position to answer the demand for love unconditionally, because she too is divided. The result of this split between need and demand is an insatiable leftover, which is desire itself.
Need is thus an intermittent tension which arises for purely organic reasons and which is discharged entirely by the specific action corresponding to the particular need in question. Desire, on the other hand, is a constant force which can never be satisfied, the constant 'pressure' which underlies the drives.
A Pre-Linguistic Need
This account presents in chronological terms what is in fact a question of structure. In truth, it is not the case that there first exists a subject of pure need which then attempts to articulate that need in language, since the distinction between pure need and its articulation in demand only exists from the moment of its articulation, by which time it is impossible to determine what that pure need could have been. The concept of a pre-linguistic need is thus merely a hypothesis, and the subject of this pure need is a mythical subject; even the paradigmatic need of hunger never exists as a pure biological given, but is marked by the structure of desire. Nevertheless, this hypothesis is useful to Lacan for maintaining his theses about the radical divergence between human desire and all natural or biological categories.