The Mysterious Universe is a popular science book which begins with a full-page citation of the famous passage in Plato's Republic, Book VII, laying out the allegory of the cave. The book made frequent reference to the quantum theory of radiation, begun by Max Planck in 1900, to Albert Einstein's general relativity, and to the new theories of quantum mechanics of Heisenberg and Schrödinger, of whose philosophical perplexities the author seemed well aware. This book is based upon the conviction that the teachings and findings of astronomy and physical science are destined to produce an immense change on our outlook on the universe as a whole, and on views about the significance of human life. The author contends that the questions at issue are ultimately one for philosophical discussion, but that before philosophers can speak, science should present ascertained facts and provisional hypotheses. The book is therefore written with these thoughts in mind while broadly presenting the fundamental physical ideas and findings relevant for a wider philosophical inquiry.