 Immunology steps into the diachronic identity debate with Burnet’s hypothesis that during embryonic development, the immune system learns to tolerate a defined set of molecules as “self” (Burnet 1959: 59). He proposed that autoreactive immune cells (lymphocytes) were purged during embryonic development to leave only those lymphocytes that ignore specific antigens encountered during this early stage (Burnet 1957). Defined negatively by a gap in the lymphocyte repertoire, immunological selfhood was assumed to persist in the adult as an invariant molecular ‘essence’ of the organism, analogous to an unwavering psychological ego (Burnet 1959; Tauber 1994: 194). As observed by Pradeu and Carosella, this self/nonself model supported a substantialist view of identity in so far as it presupposed that the immune system ensures integrity of a preserved metaphysical core (Pradeu and Carosella 2006a: 246).