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.
[citation needed] The most famous defender of necessitarianism in the history of philosophy is Spinoza.
His brief Inquiry Concerning Human Liberty (1715) was a key statement of the necessitarianist standpoint.
The Century Dictionary defined it in 1889–91 as belief that the will is not free, but instead subject to external antecedent causes or natural laws of cause and effect.