Necessarianism vs. Necessitarianism — What's the Difference?

An extreme form of determinism that holds that all phenomena, including the will, are subject to immutable rules of cause and effect; necessitarianism.

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.

The doctrine of philosophical necessity; necessitarianism.

The doctrine holding that all being is necessarily in the state that it is and denying any notion of possibility.

(metaphysics) necessarianism

The doctrine of philosophical necessity; the doctrine that results follow by invariable sequence from causes, and esp. that the will is not free, but that human actions and choices result inevitably from motives; determinism.