For much of the 20th century, the question of building mind from matter was considered so difficult that consciousness was a topic non grata. The Behaviorist school of thought held that consciousness was an illusion: The brain was to be understood simply as an input-output black box of reflexes, and that’s where the investigation must end. Private, subjective experience, immeasurable by definition, was not on the scientific map. But the past several decades have seen a shift. Giants of biology, such as the late Francis Crick, began to take the problem seriously. They asked if consciousness could be addressed just like any other scientific problem. Once they started down that road, some basic insights began to emerge.

Onto this stage steps Giulio Tononi, a pioneering neuroscientist at the University of Wisconsin at Madison. In “Phi: A Voyage From the Brain to the Soul,” Tononi expounds a theory of consciousness that he has been developing for well over a decade.