What counts as an individual in the living world? What does it mean for a living thing to remain the same through time, while constantly changing? These questions are the province of immunology, one of the most dynamic fields in biology. Immunology answers these questions with its theory of "self" and "nonself" which has dominated the field since the 1940s. Thomas Pradeu argues that this theory is inadequate, because immune responses to self constituents and immune tolerance of foreign entities are the rule, not the exception. Instead Pradeu advances an alternative theory, the continuity...