Chicago and San Francisco, 1916–2007

Benjamin Libet

The brain begins to prepare a voluntary movement about a third of a second before you are aware of deciding to make it. If conscious will has any power, Libet suggested, it may be the power to say no.

The view

Libet was a neurophysiologist at the University of California, San Francisco, who spent his career on the timing of conscious experience. In 1965 two German researchers, Hans Kornhuber and Lüder Deecke, had found a slow build-up of electrical activity over the scalp before voluntary movements, which they called the Bereitschaftspotential, or readiness potential. Libet asked when, in relation to that build-up, people become aware of their intention to move.

In experiments published in 1983, people sat in front of a clock whose spot went round once every 2.56 seconds. They flexed a wrist whenever they felt the urge, and afterwards reported where the spot had been when they first felt it. The readiness potential began, on average, about 550 milliseconds before the movement. The reported urge came about 200 milliseconds before it. So the brain seemed to start the action some 350 milliseconds before the person knew they had decided.

The argument

Conscious will may not start a voluntary act, but it could still control it, by allowing it to go ahead or vetoing it. After Libet, "Unconscious cerebral initiative and the role of conscious will in voluntary action" (1985)

Libet did not conclude that free will is an illusion. The 200 milliseconds between awareness and movement, he argued, leave time for the conscious mind to stop the act. He called this the veto, and others nicknamed it "free won't". People in his experiments reported sometimes feeling an urge and suppressing it. Try both endings below.

The experiments have been repeated, refined and argued over ever since. In 2008 a team led by John-Dylan Haynes used brain scanning to predict, a few seconds in advance and somewhat better than chance, which of two buttons people would press. Critics, including Alfred Mele, have questioned what the readiness potential means, how reliable the reported times are, and whether arbitrary wrist-flicking says anything about real choices. You can try Libet's clock yourself in the lab.

Where he meets Dennett

Libet was not a free will denier, and in that he and Dennett were on the same side against those who later cited him. Both thought the experiments showed something real and interesting about the timing of awareness.

Where they part

On what the timing means. Dennett argued, with Marcel Kinsbourne, that there is no single moment at which a process in the brain "becomes conscious", because there is no place in the brain where everything comes together to be watched. So the question Libet asked, exactly when did you decide, has no precise answer to find. More importantly, Libet's picture still has a small conscious self inside, arriving late and then allowed a veto. Dennett thought that self was a fiction. You are not the part that arrives at minus 200 milliseconds. You are the whole system, including the processes that began at minus 550, and they were your decision too.

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Further reading

• Benjamin Libet, Mind Time: The Temporal Factor in Consciousness (Harvard, 2004).
• Walter Sinnott-Armstrong and Lynn Nadel (eds), Conscious Will and Responsibility (Oxford, 2011): essays on Libet's legacy.
• On Libet and consciousness, see the lab on our sister site, Philosophy of Mind.