The life of Jean-Dominique Bauby took a dramatic turn on Friday 8 December 1995. On that day, he had a cerebrovascular accident that left him in a coma. When he woke up twenty days later, his body did not respond, his brainstem was damaged beyond repair. Able to move only his left eyelid, Jean-Dominique patiently wrote a whole book, The Diving Bell and the Butterfly, where he describes his experience.

In the past, it was known as a “massive stroke,” and you simply died. But improved resuscitation techniques have now prolonged and refined the agony. You survive, but you survive with what is so aptly known as “locked-in syndrome.” Paralyzed from head to toe, the patient, his mind intact, is imprisoned inside his own body, unable to speak or move. In my case, blinking my left eyelid is my only means of communication.

Up until his death, three days after the publication of the book, there was no doubt for anyone that Jean-Dominique had retained every aspect of his consciousness. There was no doubt that this motionless mass was genuinely conscious.

The Gaia hypothesis

But how do we know that someone is conscious, what...