Replacement is always dramatic. How does one thing or person appearing in the place of another - the second baby, the new lover, the maternity cover or 'surrogate' mother - constitute a threat to the earlier occupant of that place? Or is the superseding figure haunted by the one for whom they substitute? What happens psychologically or socially to the pair-structure which is thus up-ended? Whom do we blame, whom do we hate, when we find ourselves in this drama? What do we want when we imagine ourselves to be irreplaceable? How might the idea of uniqueness lead to an unstoppable chain of repetitions?