Pierre-Simon Laplace's famous thought experiment, often called Laplace's demon, imagines an intelligence vast enough to know the exact position and momentum of every particle in the universe at a single instant, together with all the laws of nature governing their motion; for such an intelligence, he argued, nothing would be uncertain, and both the future and the past would lie open to a single calculation, since every subsequent state of the universe follows necessarily from its prior state plus the laws. This is determinism stated at its strongest and clearest: not merely that events have causes, which almost no one denies, but that the causal chain running from the distant past through the present moment fixes exactly one possible future, with no branching points anywhere at which things could genuinely have gone otherwise.
Ancient atomism had already reached a related picture on different grounds, with Democritus holding that atoms move through the void according to fixed necessity rather than chance, though later atomists, including Epicurus, controversially introduced a small random swerve, the clinamen, specifically to make room for something other than strict necessity. Quantum mechanics has since complicated Laplace's clean picture considerably, since at the quantum level events appear genuinely probabilistic rather than fixed, though whether this genuinely restores freedom or merely adds randomness to the causal chain, which is not obviously the same thing as freedom, remains a live and unresolved dispute.