A philosophical doctrine holding that only what is actual exists, and that potential things are not real; also a contemporary metaphysics rejecting the real existence of possible worlds.
From 'actual' plus '-ism' (a doctrine or system). Developed in modern philosophy as a counterpoint to modal realism and theories of possible worlds.
Philosopher David Lewis championed 'modal realism' (possible worlds are real), so 'actualism' emerged as the opposing view—imagine philosophical chess where actualists say 'only this board matters,' not infinite alternate boards.
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