This book from 1920 consists of the Tarner Lectures in the philosophy of science and features Whitehead's assessment of the impact of Einstein's theories on nature. He argues for taking events and the process of becoming as the starting points for analyzing reality. Whitehead criticizes the idea of nature as a mere aggregate of independent entities, each capable of isolation. According to this idea, by their accidental relations entities form the system of nature. In this theory space might exist without time, and time without space. The relational theory of space is an admission that space without matter or matter without space cannot exist. Our knowledge of nature is an experience of activity or passage. Events are active entities; their relations with one another differentiate into space-relations and time-relations. But this differentiation is comparatively superficial, since time and space are each partial expressions of one fundamental relation between events, which is neither spatial not temporal. Whitehead calls this relation Extension: it is the relation of including and does not require spatio-temporal differentiation. An integral addition to any New Thought library and a great insight into the theories of nature and the universe. Cover photography by Paul Spremulli.