Determinism is the thesis that the state of the world at any one time, together with the laws of nature, fixes its state at every other time, and almost every popular objection to it is aimed at something else.
The previous lesson left the thesis as a placeholder. This one makes it precise, because four different claims travel under the name, at least two of them are false, and an argument that beats one of the false ones has beaten nothing. A reader who can state determinism exactly is already ahead of most published commentary on the subject.
Laplace's version
The canonical statement comes from Pierre-Simon Laplace, in the Essai philosophique sur les probabilités of 1814. Imagine, he wrote, an intelligence that at a given instant knew all the forces animating nature and the position of every item that composes it, and that was vast enough to analyse this data. For such an intelligence nothing would be uncertain, and the future, like the past, would be present to its eyes.
Three features of that sentence are worth extracting, because they are usually lost in the retelling.
The first is that the intelligence is a rhetorical device. Laplace's claim is about the world, not about what any knower can achieve. Whether such a being could exist is irrelevant to whether the world has the property he describes, and the fact that it plainly could not exist is not an objection to the thesis.
The second is that the entailment runs both ways. "The future, like the past" is not decoration: if a complete state plus the laws fixes every later state, the same machinery run backwards fixes every earlier one. Determinism is time-symmetric, so the past is exactly as fixed by the present as the future is. Anyone who finds the fixed future oppressive and the fixed past unremarkable should notice that the thesis treats them identically, and ask what work the asymmetry in their reaction is doing.
The third is that Laplace was writing an introduction to a book about probability. He did not think probability was a rival to determinism; he thought it was the science of our ignorance of a determined world, which is why the two subjects sat happily in one volume.
The modern statement
Stripped of the demon, determinism is a claim about entailment. Let be the complete set of laws of nature and let be a complete description of the world's physical state at time . Then determinism says that for any two times and ,
where is entailment in the sense of the logic course: there is no possible world in which the laws hold and the state at is as described and the state at differs. An equivalent and often more useful formulation is due to Peter van Inwagen: if two possible worlds obey the same laws and agree completely at any one instant, they agree at every instant.
Notice what is packed into "complete". The state must fix everything relevant, not just the large objects: a description that omits one photon is not , and determinism claims nothing about it. Notice also that this is a claim about a total state of the whole world at an instant, not about isolated systems. Nearly every real subsystem is open to influence from outside, so a determinist expects local behaviour to be unpredictable in practice.
And notice what the formula does not contain: any mention of agents, of choices, of compulsion or of causation. It is a relation between descriptions and laws. Every argument from determinism to the absence of free will therefore needs further premises, and the whole of the incompatibilist case consists of supplying them.
It is not fatalism
Fatalism is the doctrine that some outcome will occur no matter what you do. It has a famous ancient form, the Idle Argument reported by Cicero and by Origen: if you are fated to recover from this illness, you will recover whether or not you call the doctor, and if you are fated not to recover, you will not recover whether or not you call, so either way calling is pointless.
Determinism says the opposite of this. On a deterministic picture the doctor matters enormously: whether you recover depends on whether antibiotics reach the infection, which depends on whether the doctor is called, which depends on your deliberating and reaching for the telephone. Determinism says that your deliberating was itself fixed by earlier conditions. It does not say that the deliberating is idle. It says it is one link in a chain, and removing a link changes the outcome.
The distinction is easy to state and surprisingly hard to hold on to, because determinism produces the same feeling as fatalism. But the feeling misdescribes the thesis. Under determinism your choices are causes of what happens; under fatalism they are irrelevant to it. The Chrysippan reply to the Idle Argument, from the third century BC, is exactly this point: recovering may be co-fated with calling the doctor, so the fated outcome comes about through your action rather than around it.
Example. During an epidemic someone reasons: "If I am going to catch it, I will catch it whatever I do; if I am not, I will not. So there is no point wearing a mask." Diagnose the error, then say what a determinist should say instead.
The error is fatalism, and specifically the assumption that the outcome is fixed independently of the intermediate steps. It is not: catching the disease depends on the viral dose reaching the airway, which depends on the mask. A determinist says that whether you wear a mask is itself fixed by prior conditions, including this argument if you find it convincing, and that the outcome is fixed partly through the mask. Both branches of the fatalist's dilemma are false, since neither antecedent is settled independently of what you do. Notice that the argument would be equally bad in an indeterministic world, which is a sign that determinism was never the problem with it.
Now you. A soldier says there is no point taking cover, because either the bullet has your name on it or it does not. What is the closest true claim in the neighbourhood of what he is saying?
Answer
That whether he takes cover is, on the deterministic picture, already fixed by the state of the world before the battle. That is a different claim, and it does not license the inference he draws, since one of the things fixed by prior conditions is whether he takes cover, and one of the things fixed by whether he takes cover is whether the bullet hits him. The fatalist argument needs the outcome to be settled independently of the action, and determinism never gives it that. It is worth adding that his premise, that the outcome is already fixed, is compatible with the outcome being fixed as "hit, because he did not take cover".
It is not predictability
The second confusion is between a property of the world and a property of us. Chaos theory made the difference vivid.
In 1961 Edward Lorenz was rerunning a numerical weather model and typed a value from a printout, 0.506, instead of the stored 0.506127. The difference is about one part in four thousand. The new run tracked the old one for a while and then diverged completely, and the paper he wrote in 1963, "Deterministic Nonperiodic Flow", named the phenomenon: a deterministic system with sensitive dependence on initial conditions.
The point for our purposes is in the title. Lorenz's system of three equations is perfectly deterministic. It is also unpredictable beyond a horizon, because any error in the starting state grows exponentially. For the real atmosphere, errors at the scales that matter double in something like a day and a half, so an initial uncertainty of one part in ten thousand grows to order one in about twenty days, and both Lorenz's own estimates and modern reruns of the calculation put the intrinsic limit on weather forecasting at roughly two weeks. No better computer or denser network of instruments removes that limit, since it comes from the dynamics rather than from the instruments.
So determinism does not imply predictability. The implication also fails in the other direction: many indeterministic systems are extremely predictable in aggregate, which is why insurers can price a policy for a population without knowing anything about an individual.
This matters for the free will argument, because a great deal of loose writing treats "your behaviour cannot be predicted" as though it established freedom. It establishes nothing of the sort. A hurricane's path cannot be predicted either.
Example. A coin toss is the standard picture of a chance event. Is it one?
Not in any sense that involves indeterminism. A tossed coin is a rigid body obeying Newtonian mechanics, and its outcome is fixed by the initial spin, upward velocity and orientation of the hand. Persi Diaconis and colleagues built a machine that tosses a coin to the same outcome every time, and their 2007 analysis of the physics predicted that an ordinary human toss should land with the same face upward as it started slightly more often than half the time, at about 51 percent, because the coin precesses about an axis that keeps one face favoured. A team led by Frantisek Bartos tested it in 2023 with 350,757 recorded flips and found the same-side proportion to be 0.508, an excess of roughly 2,800 flips over what a fair process would give. The randomness of a coin is entirely our ignorance of the initial conditions, exactly as Laplace said, and calling an outcome random is a statement about the tosser rather than about the coin.
Now you. Someone concludes from the coin result that human decisions are likewise determined, since brains are physical systems. What is right and what is missing in that inference?
Answer
What is right is the general point that unpredictability is no evidence for indeterminism, so pointing at the difficulty of forecasting a person establishes nothing. What is missing is any argument that the brain resembles a coin in the relevant respect. A coin is a rigid body with six degrees of freedom whose governing equations are classical to an excellent approximation; a brain has roughly 86 billion neurons and is warm enough for thermal noise to matter at the level of single channels, so whether its trajectory is fixed by its state is exactly the open question the next lesson takes up. The inference reaches a conclusion that may well be true by a route that does not support it, which is a defect worth catching even when you agree with the conclusion.
It is not the claim that everything has a cause
The third confusion is with universal causation, the thesis that every event has a cause. These come apart in both directions.
An indeterministic world can be full of causes. If a radioactive nucleus has a fifty percent chance of decaying in the next hour and does decay, its decay was undetermined, and yet there is nothing wrong with saying that the instability of the nucleus caused it. Probabilistic causation is a perfectly respectable notion, and most of medicine works in it: smoking causes lung cancer without determining it.
A deterministic world need not be full of causes in the ordinary sense either. Fundamental physics is written in laws of evolution, not in cause-and-effect pairs, and Bertrand Russell argued in 1913 that the notion of cause survives in science only as a relic. Whether or not that is right, the point is that determinism as stated says nothing about causes, and an argument that leans on "everything is caused" is leaning on a different thesis that will need its own defence.
It is not compulsion
The fourth confusion is the most rhetorically powerful and the least defensible. Determinism is regularly presented as though the laws of nature force us to act, so that a determined agent is a puppet on strings.
Laws of nature are not agents. They do not push, command or constrain: on the standard reading they are descriptions of regularities the world in fact exhibits, and on the strongest realist reading available they are relations between universals, which still do not exert pressure on anyone. A puppet is unfree because a puppeteer, an agent with its own aims, moves it against or without its own. There is nothing playing that role in a determined universe. The image imports an oppressor into a picture that contains only the world doing what worlds do.
This is not a knockdown reply, and it should not be taken for one. The incompatibilist argument in a later lesson concedes the whole point and then reconstructs the problem without any hint of compulsion, using nothing but the fixity of the past and the laws. That is why the argument is good. But the puppet is the version most people carry around, and it should be put down before going further.
Example. Sort these four claims: (a) the state of the world in 1900 plus the laws entails the state of the world today; (b) whatever will happen will happen, so effort is pointless; (c) a sufficiently powerful computer could predict your next sentence; (d) you are compelled by physics to do what you do.
Only (a) is determinism. (b) is fatalism, which determinism denies, since under determinism effort is one of the things that fixes outcomes. (c) is a claim about predictability, which does not follow from determinism because of sensitive dependence, and which in any case is impossible for a predictor inside the system it is predicting. (d) is the compulsion picture, which adds a coercer to a thesis that mentions none. Someone attacking free will properly needs (a) and nothing else, and someone defending it against (b), (c) or (d) has not yet engaged.
Now you. A physicist objects that quantum mechanics has refuted determinism, so the whole debate is over. What has gone wrong with the objection, before we even get to the physics?
Answer
Two things. First, the objection assumes that refuting determinism restores free will, which requires the incompatibilist conditional plus the assumption that undetermined events are under an agent's control, and the second of those is exactly what hard incompatibilists deny. Second, the argument for incompatibilism does not actually need strict determinism: a world in which the past and the laws fix objective chances, and nothing else about you influences those chances, is as hostile to the ability to do otherwise in the relevant sense as a fully determined one. What quantum mechanics actually says, and how much of it survives the two live interpretations, is the next lesson.
Near-determinism is enough
That last point deserves stating on its own, because it decides how much the physics can matter.
Suppose the world is deterministic except for genuinely chancy events at the smallest scale, which is roughly what a standard reading of quantum mechanics gives. Ted Honderich called this near-determinism, and it changes nothing about the argument. Rewrite the thesis with chances: the state at plus the laws fixes the objective probability of every later state. Now ask whether it is up to you what those chances are. It is not, since they are fixed by conditions in place before you were born, and adding a random element to a process you do not control does not hand you control of it.
An analogy makes the shape clear. If a decision were settled by a coin flip inside your head, the outcome would be undetermined and would not be up to you in any sense worth having. The libertarian's task, in a later lesson, is to explain how the indeterminism could be located and structured so that it yields control rather than noise. It is a genuinely difficult task, and the best attempts at it are serious philosophy rather than hand-waving.
So the thesis is now clean. Determinism is an entailment claim about complete states and laws, distinct from fate, from predictability, from causation and from compulsion, and it has a weaker relative that supports the same arguments. The obvious next question is whether it is true, and the answer physics gives is more interesting and much less decisive than either side of this debate usually reports.