The book describes the Integrated Information Theory (IIT), a quantitative, rigorous, consistent and empirically testable theory that starts with experience and proceeds to the underlying neuronal mechanisms. “Integrated information” is a mathematical measure quantifying how much any system, no matter how simple or how complex, is determined by its past state and how much it can influence its future, its intrinsic causal power. Any system that has this potential is conscious. The larger the system’s integrated information, referred to by the Greek letter phi (pronounced fi), the more conscious the system is. If something has no causal power upon itself, such as the neural networks that underlying machine learning, its phi is zero. It doesn’t feel like anything to be this thing.