The original discovery that an electrical potential (of just a few microvolts - μV) is visible in the brain long before the subject flexes a finger was made by Kornhuber and Deecke (1964). They called it a "Bereitschaftspotential" or readiness potential.

The neurobiologist John Eccles speculated that the subject must become conscious of the intention to act before the onset of this readiness potential. Benjamin Libet decided to test Eccles's idea.

Libet's clock. Click to enlarge

Libet's 1983 experiments measured the time when the subject became consciously aware of the decision to move the finger. Libet created a dot on the screen of an oscilloscope circulating like the hand of a clock. The subject was asked to note the position of the moving dot when he/she was aware of the conscious decision to move a finger or wrist.

Libet's results were eagerly adopted by the deniers of free will to indicate that the mind had been made up unconsciously, long before any awareness of "conscious will."

Psychologist Daniel Wegner thinks that conscious will may be just an epiphenomenon, something that is caused by brain events, not the cause of such events. As he put it in his 2002 book The Illusion of Conscious Will,

We don't know what specific unconscious mental processes the RP might represent....The position of conscious will in the time line suggests perhaps that the experience of will is a link in a causal chain leading to action, but in fact it might not even be that. It might just be a loose end — one of those things, like the action, that is caused by prior brain and mental events.

(The Illusion of Conscious Will, MIT Press, p.55)

Does the compass steer the ship? In some sense, you could say that it does, because the pilot makes reference to the compass in determining whether adjustments should be made to the ship's course. If it looks as though the ship is headed west into the rocky shore, a calamity can be avoided with a turn north into the harbor. But, of course, the compass does not steer the ship in any physical sense. The needle is just gliding around in the compass housing, doing no actual steering at all. It is thus tempting to relegate the little magnetic pointer to the class of epiphenomena — things that don't really matter in determining where the ship will go.

Conscious will is the mind's compass. As we have seen, the experience of consciously willing action occurs as the result of an interpretive system, a course-sensing mechanism that examines the relations between our thoughts and actions and responds with "I willed this" when the two correspond appropriately. This experience thus serves as a kind of compass, alerting the conscious mind when actions occur that are likely to be the result of one's own agency. The experience of will is therefore an indicator, one of those gauges on the control panel to which we refer as we steer. Like a compass reading, the feeling of doing tells us something about the operation of the ship. But also like a compass reading, this information must be understood as a conscious experience, a candidate for the dreaded "epiphenomenon" label.

(The Illusion of Conscious Will, MIT Press, p.317)

Bernard Baars says there are two important time scales of consciousness

Sensory events occurring within a tenth of a second merge into a single conscious sensory experience, suggesting a 100-millisecond scale. But working memory, the domain in which we talk to ourselves or use our visual imagination, stretches out over roughly 10-second steps. The tenth-of-a-second level is automatic, while the 10-second level is shaped by conscious plans and goals.

(In the Theater of Consciousness, p.48)

Note also that the abrupt and rapid decisions to flex a finger measured by Libet bear little resemblance to the kinds of two-stage deliberate decisions for which we can first freely generate alternative possibilities for action, then evaluate which is the best of these possibilities in the light of our reasons, motives, and desires - first "free," then "will."

We can correlate the beginnings of the readiness potential (350ms before Libet's conscious will time "W" appears) with the early stage of the two-stage model, when alternative possibilities are being generated, in part at random. The early stage may be delegated to the subconscious, which is capable of considering multiple alternatives (William James' "blooming, buzzing confusion") that would congest the single stream of consciousness.

Alfred Mele has criticized the interpretation of the Libet results on two grounds. First, the mere appearance of the RP a half-second or more before the action in no way makes the RP the cause of the action. It may simply mark the beginning of forming an intention to act. In the two-stage model, it is the considering of possible options.

Libet himself argued that there is enough time after the W moment (a window of opportunity) to veto the action, but Mele's second criticism points out that such examples of "free won't" would not be captured in Libet experiments, because the recording device is triggered by the action (typically flicking the wrist) itself.

Thus, although all Libet experiments ended with the wrist flicking, we are not justified in assuming that the rise of the RP (well before the moment of conscious will) is a cause of the wrist flicking. Libet knew that there were very likely other times when the RP rose, but which did not lead to a flick of the wrist.

Thinkers (e.g., Daniel Wegner, Patrick Haggard) who claim that the Libet experiments prove that our conscious will and subsequent actions are caused by prior neural activity - the popular view that "my neurons made me do it" - are simply wrong.