Laplace's Demon
The thought experiment imagines an intellect that knows every state and law of a deterministic universe.
The Mechanism: Laplace imagined an intellect with complete knowledge of the positions and motions of all matter and the laws governing it. In a classically deterministic model, such an intellect could in principle infer the past and future.
The Philosophical Pivot: The thought experiment separates determinism from predictability. Even if events follow laws, finite observers may lack the information, precision, or computational power needed to forecast them.
The Horizon: Modern physics complicates the classical picture through quantum theory, while chaos can make long-range prediction practically fragile. Neither point simply resolves every philosophical question about determinism.
The demon is an idealization, not a proposed physical observer. It helps expose assumptions about whether complete knowledge is possible and what prediction would require.
Unpredictability does not automatically establish free will, just as determinism does not by itself show that responsibility is meaningless.
The distinction between what happens, what follows from laws, and what anyone can know remains essential to debates about agency.
A future may be determined by laws and still remain beyond every actual prediction.
If no one can know the future in full, what would it mean for that future to be fixed in a way that matters to choice?