Asimov meticulously tracks how science moved away from the idea of a "vital spark" (an unmeasurable soul) toward a chemical understanding of life. He explains how the discovery of urea synthesis proved that the "stuff of life" follows the same rules as inorganic matter.

Es crucial entender el alcance de esta obra. La no es un libro de texto para aprobar un examen de anatomía o bioquímica. Es un libro de contexto.