Necessitarianism is a metaphysical principle that denies all mere possibility; there is exactly one way for the world to be. It is the strongest member of a family of principles, including hard determinism, each of which deny libertarian free will, reasoning that human actions are predetermined by external or internal antecedents. Necessitarianism is stronger than hard determinism, because even the hard determinist would grant that the causal chain constituting the world might have been different as a whole, even though each member of that series could not have been different, given its antecedent causes.