This commentary of Aquinas on the first book of the Physics of Aristotle is a summary of the thought of the Pre-Socratics and of Aristotle's approach to cosmology. A unit with all cross-references in English, it clarifies the thought of the ancients and of the medieval Aquinas with regard to the philosophy of nature; it presents all of this as a basis for subsequent philosophy of science.This work can be read by the layman; it can be used as a text for an introduction to philosophy course; and it can be read with profit by one seeking a fully researched work in ancient philosophy. The work is a long overdue text for both the history of ancient philosophy and of the synthesis of ancient philosophers with that of Aquinas. It presents in English both the text of Aristotle and that of Aquinas' Commentary.