Every event has a cause. Every choice you make is the product of prior conditions: your genes, your upbringing, your brain chemistry, the laws of physics. If you could know everything about the present, you could predict the future with certainty. That is determinism, and it has troubled philosophers for millennia. If everything is determined, in what sense are we free? Can we be held responsible for actions we could not have avoided?

The ancient Stoics were determinists, though they carved out a role for human agency. Spinoza argued that freedom consists in understanding necessity, not escaping it. Laplace imagined a demon that could calculate the entire future from the present state of the universe. Quantum mechanics complicated the picture. Some interpretations introduce genuine randomness at the subatomic level. But randomness is not freedom either. Indeterminism does not restore control. Compatibilists argue that freedom and determinism are compatible: freedom means acting according to your own desires and reasoning, without external coercion. Incompatibilists disagree. Libertarians hold that free will requires a break in the causal chain.

Positions on free will

• Hard determinism. Determinism is true, free will is an illusion.
• Libertarianism. Determinism is false, free will is real.
• Compatibilism. Determinism and free will can coexist.
• Hard incompatibilism. Free will is incompatible with both determinism and indeterminism.

Determinism is not fatalism. Fatalism says outcomes happen regardless of what you do. Determinism says outcomes happen because of what you do, along with everything else.