Originating from the Rede Lecture delivered at the University of Cambridge in November 1930, 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.