 Subsequent authors argued that a hypothesis proposed for the sake of immunizing a theory from falsification could be ‘suspicious’ for various reasons, and thus could be ‘ad hoc’ in various ways. Zahar (1973) argued that a hypothesis was ad hoc1 if it had no novel consequences as compared with its predecessor (i.e. was not independently testable), ad hoc2 if none of its novel predictions have actually been verified (either because it has not yet been tested or has been falsified), and ad hoc3 