A person who believes that all events are predetermined and that people don't have free choice; one who adheres to the philosophical doctrine of determinism.
From determine + -ist (suffix marking a person who believes in or practices something). Determinism as a philosophy emerged in the 17th-18th centuries alongside mechanistic physics.
Determinists argue that since your brain is just atoms following physics, you can't truly choose—free will is an illusion, which bothers philosophers and comforts neuroscientists.
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