Libertarianism is the view that we have free will and that determinism is therefore false, and it has to explain how an undetermined decision could be more under an agent's control than a determined one rather than less.
That is a real bill, and the position is not disreputable: roughly one philosopher in five holds it, including van Inwagen, who built the argument in the fifth lesson of this course. This lesson sets out what the view has to supply, the objection that all versions face, and the two families of answer. It is the fairest test of the view, because the strongest objection to libertarianism comes from within the incompatibilist camp rather than from compatibilists.
Three things the position must deliver
A libertarian owes three separate things, and the second and third are much harder than the first.
The first is that determinism is false, which the physics lesson said is a live option and not established either way.
The second is that the indeterminism is located where decisions are made. Undetermined decay in a distant star is no help. The indeterminism has to be in the process that produces the choice, at a time when it can make a difference to the outcome.
The third, and the one everything turns on, is that the indeterminism yields control rather than noise. If the undetermined element makes the decision less predictable without making it more the agent's own, the view has purchased randomness at the cost of authorship, and randomness excuses rather than empowers. Nobody blames a person for a seizure.
The rollback argument
Van Inwagen stated the difficulty in its cleanest form in 2000, in a paper whose title is a fair summary of his own position: free will remains a mystery.
Alice is deciding whether to tell a damaging truth. She deliberates, and at time she decides to tell it. Suppose the decision was undetermined: the state of the world and the laws left both outcomes open right up to the moment.
Now imagine God rewinds the world to a moment shortly before and lets it run again, with everything exactly as it was, and does this a thousand times. Since the decision is undetermined, we should not expect the same outcome every time. Suppose she tells the truth in 726 of the replays and lies in the other 274. That distribution is all there is to say: nothing about Alice differs across the replays, since the world is in exactly the same state each time, and no fact about her, her reasons, her character or her efforts explains why this replay went one way and that one went the other.
The conclusion is uncomfortable. If in the actual world she told the truth, that was the 0.726 branch coming up rather than the 0.274 branch, and the difference between the two is not attributable to anything about her. It looks exactly like luck. And if instead the replays all came out the same way, the decision was not undetermined after all.
The argument is not a proof, but it isolates the problem precisely. Indeterminism, placed at the moment of decision, appears to insert chance exactly where control was wanted.
Example. A libertarian replies that the rollback picture misdescribes the case, because in every one of the thousand replays Alice acts for reasons she has, and does what she wants. Is that a good reply?
It is a real observation and an incomplete reply. It is true that Alice has reasons for both options, since that is why the decision was hard, and true that whichever she takes she takes for a reason. What the argument asks is a different question: what explains the pattern of 726 to 274? Not her reasons, since they are identical across replays. Not her character, likewise. Nothing about her at all, since everything about her is held fixed by the stipulation. So the reply establishes that neither outcome would be alien to her, which is worth having, and does not yet establish that she controls which one occurs. The best versions of libertarianism start from exactly this observation and try to build the rest.
Now you. Does the rollback argument work equally against a compatibilist account of decision?
Answer
No, and seeing why sharpens what the argument is. Replay a determined decision a thousand times from the same state and the same laws and the outcome is the same every time, by definition. There is no distribution to explain and no unexplained difference between replays. A compatibilist can say the outcome is fixed by the agent's reasons and character, which is precisely the connection Hume identified as the ground of responsibility. The rollback argument is therefore an argument against indeterministic accounts specifically, which is why it comes from an incompatibilist. It is not an argument that everything is fine, since the compatibilist still faces the manipulation cases of the previous lesson.
Event-causal libertarianism: Kane
Robert Kane's The Significance of Free Will of 1996 is the most developed attempt to make indeterminism yield control without invoking anything exotic.
Kane concedes that most actions are determined by the agent's character, and that this is fine: an honest person acting honestly out of settled habit is free enough, provided her character was formed by acts of the right kind. The freedom-conferring acts are the rare ones, which he calls self-forming actions. They occur when a person is genuinely torn, when two incommensurable sets of reasons pull in different directions, a businesswoman on her way to a meeting who sees an assault in an alley being his standard example.
In such a case, Kane argues, the agent makes two efforts at once. She is trying to make the moral choice and trying to make the prudential one, and the conflict is realised in the brain as competing processes that are indeterministic because they are sensitive to noise amplified through chaotic neural dynamics. Whichever effort succeeds, it succeeds by the agent's own effort of will, for her own reasons, and she endorses the outcome as hers. She therefore has plural voluntary control: whichever way it goes, she wanted it, tried for it, and is responsible for it.
The move is genuinely clever, because it converts the indeterminism from an intruder into a feature of a divided will. The residual objection, pressed hardest by Alfred Mele, is that it does not remove the luck but relocates it. Grant that Alice is responsible either way. Which effort in fact succeeds is still settled by nothing about her, and the difference between the world where she helps and the world where she goes to her meeting is still, in the rollback sense, a matter of chance. Kane's answer is that responsibility does not require an explanation of that difference, only that both outcomes be willed and endorsed. Whether that is enough is where the argument currently stands.
There is also an empirical bill. Kane's picture requires indeterminism to be amplified to the level of competing neural processes, and no such amplification has been demonstrated. The physics lesson set out why the candidate mechanisms are unpromising at body temperature, and Kane has always been candid that his account is a proposal about how the neuroscience might turn out rather than a report of it.
Agent-causal libertarianism
The other family takes the harder metaphysical route: the cause of a free decision is not a prior event at all, but the agent, as a substance.
The idea is old. Thomas Reid stated it in the Essays on the Active Powers of Man of 1788, and Roderick Chisholm revived it in 1964 in a lecture with a famous line: each of us, when we act, is a prime mover unmoved, causing events without being caused to cause them. Timothy O'Connor and Randolph Clarke have developed modern versions.
The attraction is that it answers the rollback argument directly rather than living with it. In the replays, what explains why Alice tells the truth this time is that she, the agent, exercised her causal power that way. The explanation terminates in a substance rather than in a further event, so there is no unexplained difference between events to be worried about.
The costs are three, and they are steep.
The first is that agent causation is unlike any causation elsewhere in nature. Every other causal relation science describes holds between events or states, and a relation whose first term is a persisting thing rather than an event is a new category admitted for one purpose.
The second is the timing problem. If Alice's causal power is not itself triggered by any prior event, what accounts for its being exercised at 3.42 in the afternoon rather than a minute earlier or later? Answering "she exercised it then" restates the fact; answering that a prior event prompted the exercise reintroduces event causation at the crucial point.
The third is fit with the brain. A decision is realised in neural activity, so the agent's causing must show up as neurons firing that would not otherwise have fired, or as chances being altered. Clarke's version accepts this and says agent causation works alongside event causation, adding a causal contribution within the space the indeterminism leaves. That is coherent, and it makes the theory empirically committed: there would be a real difference between a brain in which an agent acts and one in which the same physical antecedents run their course.
A third and smaller family, non-causal libertarianism, defended by Carl Ginet and Hugh McCann, denies that a free action needs any causing of the decision at all: basic actions have an intrinsic character of being an agent's own doing, and asking what caused them is a mistake. The standard objection is that this names the phenomenon rather than explaining the control, and few have been persuaded.
Example. Apply the timing problem to a mundane case: you decide, at some moment, to stand up and make tea. What must the agent-causal theorist say?
They must say that the standing up is caused by you, as a substance, and not by any prior event, while conceding that the moment at which the power is exercised has no event-causal explanation. Reasons can still be cited, since agent causationists usually allow that reasons make an exercise of the power more or less likely without necessitating it, but as soon as reasons raise probabilities we are back to a chancy connection between antecedent states and the decision, and the rollback argument applies to the residue. The honest statement of the position is that it purchases control by adding a primitive, and the question for a reader is whether the phenomenon to be explained is strange enough to justify a primitive. Note that this is exactly the sort of trade philosophers make elsewhere, and not automatically illegitimate.
Now you. What empirical result would embarrass a libertarian, and what would embarrass a compatibilist?
Answer
A libertarian would be embarrassed by a demonstration that decision-making in the brain is effectively deterministic at the relevant scale, since noise averages out across large populations of neurons, or by a demonstration that no mechanism amplifies microscopic indeterminacy to the level of competing intentions. Neither has been shown, and both are the sort of thing that could be shown. A compatibilist is much harder to embarrass, since the position makes no claim about physics; what would trouble them is evidence that the excusing practices they claim to be describing do not in fact track reasons-responsiveness and constraint, and here experimental philosophy has produced results in both directions. The asymmetry is worth noticing: it is a cost to libertarianism that it takes an empirical risk, and a cost to compatibilism that it takes almost none.
The argument from how it feels
Most people who hold a libertarian view hold it because of how deciding seems from the inside, and that deserves an honest hearing rather than a dismissal.
The claim has a distinguished history. Descartes said the freedom of the will is known without proof by experience of it. Thomas Reid built his philosophy on the deliverances of common sense, of which the sense of active power is one. Samuel Johnson gave the popular version in 1778: we know our will is free, and there's an end on't.
The evidence against treating the feeling as a report is now substantial, and it is not the Libet material. Daniel Wegner assembled it in The Illusion of Conscious Will in 2002. In the experiment he ran with Thalia Wheatley in 1999, two people jointly moved a computer mouse over a screen of objects; when a subject heard the name of an object shortly before the mouse stopped on it, they reported having intentionally stopped it there, even though the movement had in fact been made by a confederate. The sense of having willed an action can be produced by a thought that merely preceded the action and matched it.
A second body of work points the same way. Patrick Haggard and colleagues reported in 2002 that when a voluntary action produces a tone, people perceive the action as occurring later and the tone as occurring earlier than they really do, each by tens of milliseconds, so that cause and effect are pulled together in experience. This intentional binding effect is absent for involuntary movements produced by magnetic stimulation. The sense of agency is therefore something the brain constructs and can construct wrongly, rather than a channel through which the will is directly observed.
None of this refutes libertarianism, and it is important to say why not. The theory is a claim about the causal structure of decisions, and the fallibility of introspection about agency is a claim about how well we detect that structure. What the evidence does remove is the argument from obviousness, which was the position's main popular support. A libertarian who wants the phenomenology as evidence has to explain why a faculty demonstrably prone to false positives should be trusted on the one occasion when metaphysics depends on it.
Example. A libertarian argues: I directly experience the openness of my future when I deliberate, so at least one undetermined decision has been observed. Assess the argument.
The premise is a report about experience, and the conclusion is a claim about the world, so the argument needs a bridge, namely that this kind of experience is reliable about this kind of fact. That bridge is what the evidence undermines. What deliberation actually presents is that you do not yet know what you will do, which is true, and compatible with the outcome being fixed: a chess engine calculating its move does not know its move until it finishes, and nothing follows about indeterminism. The experience of openness is the experience of ignorance about your own future decision, which every party to this dispute agrees is real. Note that this is one of the rare arguments in the subject that both compatibilists and hard incompatibilists reject for the same reason.
Now you. Does the same reply work against the experience of effort in a difficult decision, which Kane appeals to?
Answer
Less well, and Kane's appeal is more careful for exactly that reason. He is not claiming that introspection reveals indeterminism; he is claiming that the phenomenology of a torn decision, with genuine effort exerted in two directions at once, is evidence about the structure of the underlying process, namely that two competing processes are running rather than one. That is a claim a neuroscientist could investigate, and it is meant to be underwritten by the mechanism rather than by the feeling. The correct objection to it is not that introspection is unreliable but the one raised earlier: even granting the two efforts, nothing about the agent explains which one succeeds.
The state of the position
Libertarianism is a minority view among philosophers and probably the majority view outside philosophy, which is an unusual combination and tells you something about how the position gets its support. What people report about their own decisions, that they could have done otherwise and that the choice was up to them, is exactly what the view says.
Its serious defenders are not naive about the difficulty. Van Inwagen accepts the Consequence Argument, believes we have free will, concludes that determinism is false, and writes that he has no idea how an undetermined choice can be an agent's own. That combination is intellectually honest and uncomfortable, and it is the position of somebody who is convinced by an argument they cannot complete.
There is one more line of attack, and it is the one that cuts deepest, because it applies to libertarians and compatibilists alike. Whatever a free decision requires, it will issue from the person you are, and you did not make yourself. The causes of your character reach back through your upbringing to your genes to conditions you had no part in. The next lesson but one turns that thought into an argument, with an empirical literature attached, and it is the strongest thing in this course. Before that, there is a body of evidence that is famous for settling the question and does not: what happens in a brain in the half-second before a movement.