The idea behind this book is that developing a conception of the physical world and a conception of mind is impossible without the exercise of agency, meaning "the power to alter at will one's perceptual inputs." The thesis is derived from a philosophical account of the role of agency in knowledge - the first time this has been attempted in the context of developmental psychology.

The book is divided into three parts. In Part One, Russell argues that purely "representational" theories of mind and of mental development have been overvalued, thereby clearing the ground for the book's central thesis. In Part Two, he proposes that, because objective experience depends upon the experience of agency, the development of the "object concept" in human infants is grounded in the development of executive-attentional capacities.

In Part Three, an analysis of the links between agency and self-awareness generates an original theory of the nature of certain stage-like transitions in mental functioning and of the relationship between executive and mentalising deficits in autism. The book will be of particular interest to students and researchers in cognitive-developmental psychology, to philosophers of mind, and to anybody with an interest in cognitive science.