In the s the debate acquired laboratory data, and the first results looked devastating for the intuitive picture of a self that initiates action.
Libet's experiment
Benjamin Libet asked volunteers to flex a wrist whenever they felt like it, with no planning. He recorded their brain activity, and had them note the position of a fast-moving dot on a clock face at the moment they first became aware of the urge to move.
Brain activity ramped up around milliseconds before the movement. The conscious urge was reported only about milliseconds before it. The brain appeared to be preparing the action roughly a third of a second before the person knew they had decided.
Libet himself did not conclude that free will is an illusion. He noted that the gap left room for conscious veto, and suggested that what we have may be less a free will than a free won't.
What came later
Later work using brain imaging pushed the lead time much further. Patterns in the frontal cortex predicted which of two buttons a participant would press up to several seconds before they reported deciding, though the predictions were only modestly better than chance.
The objections
The experiments are real, but what they show is contested.
The tasks are arbitrary. Deciding to twitch a wrist at no particular moment for no particular reason has little in common with deciding to change career or return a lost wallet, and it is not obvious the same machinery is involved.
The timing rests on introspection. Participants report when they noticed an urge by reading a clock, and the accuracy of that report is exactly what one might doubt.
The ramping signal may not be a decision at all. On one account it reflects ordinary background fluctuation in neural activity, with the movement triggered when the noise happens to cross a threshold. If so, it is not the brain having secretly decided; it is the brain drifting until something tips.
What the science can and cannot settle
There is a deeper reason not to expect an experiment to end this. That conscious deliberation is implemented in physical brain events was never in dispute between the positions. A hard determinist and a compatibilist both expect decisions to have neural precursors, and both expect those precursors to precede awareness.
What they disagree about is whether being a physically caused process disqualifies a decision from being free, and that is a question about what "free" requires, not about when a signal appears on an electrode. The neuroscience constrains the story; it does not adjudicate it. The practical question is what any of this means for holding people responsible.