The book is divided into part one, presenting a survey and comparison of the sciences, and part two “Philosophy”, devoted to the philosophical discussion of the problems of scientific conceptions, raised in part one. In lecture one Haldane examines biology and the physical sciences, concluding that biology must be regarded an independent science, guided by a conception different from the mechanical one that is ordinarily sufficient in the physical sciences. The next four lectures “The Rise of Mechanistic Biology”, “The Fate of Mechanistic Biology”, “Vitalistic Biology”, and “Biology an Independent Science” present an account of the attempts which have been made to interpret life. On the one hand, life has been explained on the basis of ordinary physical and chemical suppositions alone; on the other hand, in addition, life has also been interpreted with the help of the assumption that living organisms are the result of a supernatural influence. In lecture 6 Haldane discusses the psychological interpretation of the visible and tangible universe of experience, and the physical or biological interpretation under consideration of conscious behaviour. Lecture 7 “Animistic Psychology”, expounding animism, the conception of a soul essentially independent of the body, followed by lecture 8 “Mechanistic and Biological Psychology” discussing the materialistic conception. Part one of the book closes with lecture 9 “Psychology an Independent Science” and lecture 10 “Defects in the Sciences”, with reflections on the importance of psychology and finally discussing the essential defects in scientific interpretation of life.

In part two of the book, Haldane presents an account of how philosophy deals with the defects of the sciences and endeavours to reach some sort of solution of them. After reflections on the need for philosophy and on biology and more abstract sciences, the relation of the external environment to the conception of organic unity is discussed. Subsequent lectures are 14 “Psychology and more Abstract Sciences”, 15 “Interest and Values”, 16 “The Unity of Experience”, and 17 “Philosophy, Religion, and Theology”. Haldane thereafter pays attention to the belief in immortality and discusses the clashes between the sciences and religious beliefs, focusing also on how far religion enters into science itself. The book has a concluding “Retrospect” lecture.