 However, argues Blackburn, ethics cannot be entirely realist either, for this would not allow for phenomena such as the gradual development of ethical positions over time.In his 1998 book, "Ruling Passions", Blackburn likens ethics to Neurath's boat, which can be changed plank by plank over time, but cannot be refitted all at once for risk of sinking.Similarly, Blackburn's theory can explain the existence of rival ethical theories, for example as a result of differing cultural traditions - his theory allows both to be legitimate, despite their mutual contradictions, without dismissing both views through relativism.