QuestionsAre we free?The brain acts before we decide
Are we free?No free will

The brain acts before we decide

The Libet experiment

Benjamin Libet 1916 · 2007The brain prepares the action before we are aware of deciding it.
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What it claims

The initiation of a voluntary act can begin unconsciously: the brain has already started the act before we are aware of deciding.

Why

The brain activity preparing the movement begins before the moment when the subject reports deciding to move.

In short

"The brain acts before we decide" is an argument by Benjamin Libet (Brain 106, 1983 (Libet, Gleason, Wright, Pearl)) on the question "Are we free?" (answer: no free will). Its claim: The initiation of a voluntary act can begin unconsciously: the brain has already started the act before we are aware of deciding. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P3, P4). AskPhilosophy presents 4 objections, 1 of them with a documented reply, and the original text behind each premise, checked word for word.

Brain 106, 1983 (Libet, Gleason, Wright, Pearl)
Valid with an implicit premise

In 1983, the neurophysiologist Benjamin Libet asked volunteers to flex their wrist whenever they felt like it. They watched a spot revolving on a clock and noted where it was when they became aware of wanting to move. Meanwhile, electrodes measured their brain activity. Result: the brain activity that prepares the movement begins before the awareness of wanting. Libet does not conclude that we have no freedom at all. He leaves a door open, the "veto": consciousness could still stop the movement. This experiment is one of the most discussed in the science of free will.

P1The moment when the subject becomes aware of wanting to move (time W) can be dated by the position of the clock spot he recalls.
P2In series where the movement is spontaneous, the preparatory brain activity (the readiness potential) begins on average about 350 ms before this time W.
P3 · implicit premiseThis readiness potential is indeed the start of the preparation of the movement, and not an activity unrelated to it.
P4 · implicit premiseA decision that appears in consciousness only after the preparation of the movement has begun cannot be what triggers that preparation.
Therefore the initiation of a voluntary act can begin unconsciously: the brain has already started the act before we are aware of deciding.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P3, the computer finds a case where brain activity precedes awareness but is not the start of the movement: the awareness of deciding could then well trigger the movement.
With P3 and P4 the conclusion follows. P1 and P2 are measurement results; P3 says what the measurements mean (the readiness potential is the start of the movement); P4 is a commonsense principle (a cause does not come after its effect). The objections mostly target P3 (Schurger) and how P4 is drawn (Mele, Dennett).
Analogy. Imagine hearing thunder a second after the lightning. You would not say the thunder caused the lightning. Libet says: the awareness of wanting comes after the brain activity begins, like thunder after lightning. But note, the image assumes that the brain activity really is the start of the movement, which Schurger disputes.

Objections

4

Each objection is a conversation: click to see who replies and where the debate stands.

Libet himself (the veto) 1999 · attacks the conclusion

The volitional process is initiated unconsciously. But consciousness could still control the outcome: it can veto the act. Free will is therefore not excluded. It would not initiate the act, but it could control its performance. Libet had already put forward this idea in a 1985 paper.

Libet replies
A
Alfred Mele 2011
No brain activity was ever recorded before a veto: with no muscle movement, the computer did not start recording. For all anyone knows, brain activity thus also preceded the urge that the subject then suppressed.
The debate goes on (our summary) The veto thus remains a hypothesis. Libet himself asks whether the veto has an unconscious origin, and he refuses to call a veto of unconscious origin real freedom. He relies on subjects who say they suppressed an urge, which is not a direct measurement.
A
Aaron Schurger, Jacobo Sitt and Stanislas Dehaene 2012 · attacks premise 3

The readiness potential may not be the start of a decision. When nothing is pressing, the precise moment of the movement is largely set by spontaneous fluctuations in neuron activity, a kind of noise that accumulates up to a threshold. By aligning all trials on the movement onset, one sees on average a gradual rise, which looks like preparation but is not.

no reply
No reply possible: objection raised in 2012, after the author's death (2007).
The debate goes on (our summary) Libet died in 2007: he could not reply. This model removes a support from Libet's argument, but it does not prove that we are free. For a determinist, neural noise fixing the moment of the movement is not a free choice either.
A
Alfred Mele 2009 · attacks premise 4

Nothing shows that the subjects had decided to move at the start of the readiness potential. What Libet measures is rather the urge or wish to move, and it is not surprising that our urges are produced by processes we are not conscious of. What matters for free will is that we then decide whether or not to act on that urge.

no reply
No reply possible: objection raised in 2009, after the author's death (2007).
The debate goes on (our summary) Mele (Effective Intentions, 2009) distinguishes an urge from a decision. Libet died in 2007, two years before this book, and so never read or commented on it.
Daniel Dennett 2003 · attacks premise 1

The subject must compare two conscious experiences: the moment he decides, and the clock position. This comparison takes time and can distort the time he reports. Dennett holds it possible that subjects made their decision consciously around 550 ms before the movement, while having the impression of making it only later.

no reply
We have not yet found a direct reply by Libet in the texts.
The debate goes on (our summary) Mele notes that the data settle neither Libet's reading (an unconscious decision around -550 ms) nor Dennett's (a conscious decision at the same time), because the potential is recorded only when a movement occurs. We found no direct reply from Libet to Dennett.
What today's voices say
Defend it
Robert SapolskyHe says people are still arguing about Libet 40 years later, but the timing of conscious intent is irrelevant. The real question is how you became the person who has that intent.
Criticize it
Daniel DennettHe rejects the conclusion drawn from neuroscience: going from a few damaged brains to everyone is 'a mathematical induction gone wrong'. Some people are wired wrong, not all.