A new theory is taking hold in neuroscience. The theory is increasingly being used to interpret and drive experimental and theoretical studies, and it is finding its way into many other domains of research on the mind. It is the theory that the brain is a sophisticated hypothesis-testing mechanism, which is constantly involved in minimizing the error of its predictions about the sensory input it receives from the world. This mechanism is meant to explain perception and action and everything mental in between. It is an attractive theory because powerful theoretical arguments support it. It is also attractive because more and more empirical evidence is beginning to point in its favour. It has enormous unifying power and yet it can explain in detail too. This book explores this theory. It explains how the theory works and how it applies; it sets out why the theory is attractive; and it shows why and how the central ideas behind the theory profoundly change how we should conceive of perception, action, attention, and other central aspects of the mind. The central argument of the book is that the simple idea of prediction error minimization offers a surprisingly good, explanatory fit with our actual perceptual phenomenology, and that it throws new light on core, intriguing aspects of the nature of mind.