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Giulio Tononi

An icebreaker

A neuroscientist proposed measuring consciousness itself with a single number, arguing any system integrating enough information has some degree of experience. In 2023 over a hundred researchers publicly labeled the theory pseudoscience; other scientists pushed back just as publicly.

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Worth knowing · 1 of 5
What does the theory actually claim?
It proposes that consciousness corresponds to a system's capacity to integrate information in a way that can't be broken into independent parts, measured by a value researchers call phi. The higher that value, the theory holds, the richer the experience.
Worth knowing · 2 of 5
What's the strangest implication of the theory?
Taken literally, it implies that even simple systems with enough internal integration, in principle including some non-biological ones, could have a small degree of experience. Critics call this implausible; supporters call it an honest consequence of taking the theory seriously.
Worth knowing · 3 of 5
What happened in the 2023 controversy?
More than 100 researchers signed an open letter calling the theory pseudoscience, arguing it makes claims that can't be tested and had been overstated in the media. Other prominent scientists and philosophers publicly disputed the label as unfair and damaging to the field.
Worth knowing · 4 of 5
How did its developer respond to being called pseudoscience?
He and his co-authors published a formal reply arguing the theory makes testable predictions and has already guided experiments with brain-injured and anesthetized patients. The exchange was later published alongside the letter in a major journal.
Worth knowing · 5 of 5
Has this theory been put to any practical use?
A related measurement tool inspired by the theory has been tested in research settings to help detect signs of covert awareness in some unresponsive patients. Whatever the dispute over the theory's philosophy, that clinical research has been reported in peer-reviewed journals.
Words to know
Phi (Φ)The theory's proposed numerical measure of how much a system integrates information, meant to track degree of consciousness.
Integrated informationInformation a system generates as a whole that can't be reduced to its separate parts working alone.
PanpsychismThe view that some form of experience is present, in some degree, throughout physical reality, a comparison the theory invites though its author resists.
More words
Perturbational Complexity Index (PCI)A related bedside measure, inspired by the theory, used experimentally to gauge awareness in unresponsive patients.
A voice · 1 of 4
Giulio Tononi · Neuroscientist who developed the theory

He proposed in the early 2000s that consciousness corresponds mathematically to a system's integrated information, later publishing a formal reply to the 2023 pseudoscience letter defending the theory's testable predictions and clinical applications.

A voice · 2 of 4
Christof Koch · Neuroscientist who helped develop and champion the theory

He worked closely with Tononi to extend and popularize the theory over two decades, while also publicly conceding, after a long-running bet with a philosopher, that no neural signature of consciousness had been found by the deadline either side had set.

A voice · 3 of 4
Anil Seth · Neuroscientist who defended the theory publicly

He called the pseudoscience accusation serious and unjustified by the letter, and argued in a 2023 essay that bold theories have a right to be wrong, while not endorsing all of the theory himself.

A voice · 4 of 4
Stephen Fleming · Cognitive neuroscientist who helped organize the 2023 letter

He co-signed the September 2023 open letter, joined by roughly 124 researchers, arguing the theory had been overstated in media coverage as a tested, leading account of consciousness without matching empirical support.

A line to leave them with
One number for consciousness. A hundred scientists said not so fast.
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