Galileo Galilei investigated the universe. In Giulio Tononi’s book that I am reviewing here (and in Nature in August of 2012), Tononi imagines Galileo investigating consciousness.

In the end, consciousness is all that matters! So writes Giulio Tononi, the author of this stunningly original scientific fantasy, in a distant echo of Rene Descartes’ famous deduction. Tononi, a neuroscientist, psychiatrist and expert on sleep and consciousness, is also that rarest of modern scholars – an idealist. In this category-defying book, he presents his theory of how brain produces mind as an oneiric journey of discovery of Galileo Galilei.

In Tononi’s literary telling of this story, Francis Crick teaches Galileo basic neuroscience – such as that the brain is the seat of the mind, and that consciousness flees when neurons turn on and off together during deep sleep or seizures – as they meet scholars, scientists, doctors and artists from the Enlightenment to the modern era. It is a vast cast, including Descartes, Nicolaus Copernicus, Charles Darwin, Sigmund Freud, Marcel Proust and, eventually, Alan Turing. Along the way, Galileo negotiates some tricky concepts on a road long trodden by neuroscientists and neurologists seeking to track consciousness down to its lair in the brain. Even if we could point to this biophysical mechanism, and those nerve cells, as mediators of the phenomenal experience of red, we would still need to ask – why these particular mechanisms and neurons? Why not others? Historically, the great challenge has been to explain how consciousness emerges from highly organized matter without invoking magic, soul-stuff or exotic physics.

With the advent of Shannon’s mathematical theory of information, information being the difference that makes a difference, scholars averred a linkage between information and conscious experience, without working out what this could be and what it might imply. Tononi’s theory of integrated information does so. Proceeding from two axioms that are rooted in everyday phenomenal experience, the theory defines a measure (the eponymous phi) associated with every system consisting of causally interacting parts. This measure is high if a system constitutes a single entity above and beyond its parts (integration) and if it is endowed with a large repertoire of discriminable states (information). The more integrated information any system has, the more irreducible it is, the more conscious it is. This framework, couched in a probabilistic language, also captures the unique quality of that experience, why blue is more similar to red than to pain or to smell.

In Phi, this is conveyed through a series of dazzling thought experiments aided by cameos from Shannon and philosophers Spinoza, Leibniz and Thomas Nagel (the only living person to figure in the book). Through them, Galileo understands how the algebra of integrated information is turned into the geometry of conscious experiences, and how this links to the physiology and the anatomy of the brain. In the book’s final third, Tononi lays out the implications of his theory. He discusses a number of points about consciousness: that it ceases in death and dementia, does not require language or knowledge of self, exists in animals in graded forms and can be present, to some degree, in the fetus. Hell, Tononi emphasizes, is all in the mind. One of the most chilling characters in Phi is the Master, an amalgam of the captain in Kafka’s 1914 short story In the Penal Colony and the Grand Inquisitor from Dostoyevsky’s The Brothers Karamazov. The Master’s obsession is creating perfect never-ending pain by manipulating the brain’s informational content. In the final chapter, the Mannequin, a stand-in for Mephistopheles, throws up some logical paradoxes before leaving the dying Galileo reunited with his beloved daughter. The only character missing is Pierre Teilhard de Chardin, whose prophetic ideas prefigure some of the implications of Tononi’s theory.

Phi is quite extraordinary, defying easy categorization. In its appeal to the reader’s imagination, it resembles Edwin Abbott’s Flatland novella and Douglas Hofstadter’s Gödel, Escher, Bach. Yet its language is more poetic, full of cultural references and images – movie stills and often modified photos of artworks. Endnotes to each chapter link the allegories and metaphors Tononi uses in the text to the science. I believe that in the fullness of time, the quantitative framework outlined in Phi will prove to be correct. Consciousness is tightly linked to complexity and to information, with profound consequences for understanding our place in the evolving Universe. As Crick says to Galileo, this is a story for grown men, not a consoling tale for children.