The book is a deep dive into the "frontiers" of what humans currently understand. Grayling divides the work into three main pillars: The Past (History and Archaeology), The Universe (Science and Physics), and The Self (Psychology and the Mind). He argues that while we have learned more in the last hundred years than in all of previous history, our discoveries have only revealed a much larger "ocean of ignorance." It is a book about the thrill of discovery and the humbling realization of how much mystery remains.