In Pierre-Simon Laplace described an intellect that knew the position and momentum of every particle in the universe, along with all the laws governing them. For such an intellect, he wrote, nothing would be uncertain: the future, like the past, would be present before its eyes.
The claim
Causal determinism is the thesis that the state of the world at any moment, together with the laws of nature, fixes exactly one possible next state. There are no forks. What happens is the only thing that could have happened, given everything that came before.
Note that this is not fatalism. Fatalism says the outcome will arrive whatever you do, which is a claim that your actions do not matter. Determinism says the opposite: your actions matter enormously, they are a genuine link in the causal chain, and they are themselves fully caused. Deliberation is not bypassed; it is part of the machinery.
The threat to the will
The problem for free will is a regress. Your choice came from your reasons, character, and mood. Those came from your brain state a moment before, which came from your upbringing, your genes, the food you ate, the argument you had last week. Follow the chain back far enough and it leaves your lifetime entirely, arriving at events long before you existed.
The philosopher Peter van Inwagen made the argument precise. If our acts are consequences of the laws of nature and events in the remote past, and neither those laws nor that past is up to us, then the consequences are not up to us either. Nothing in the chain is a place where you could have inserted a different outcome.
Does quantum mechanics help?
Physics turned out not to be Laplacian. Quantum mechanics appears to be irreducibly probabilistic: identical conditions can yield different outcomes, and only the odds are fixed.
This is real, but it is a disappointing rescue. Suppose some of your neurons fire because of genuinely random quantum events. That makes your choice undetermined, but it does not make it yours. An action that issues from a chance fluctuation is not more under your control than one that issues from a cause; it is less. Randomness delivers unpredictability, which was never the thing we wanted.
The point generalises into a dilemma that shapes everything that follows. If your choices are determined, they trace to causes outside you. If they are undetermined, they are to that extent matters of chance. Either way it is hard to see where the authorship goes. That dilemma is the challenge the next lesson develops.