This edition includes five treatises by Galen, an outstanding Roman physician and philosopher of the 2nd–3rd centuries, the creator of a theoretical and practical system that became the basis for the development of medicine and natural science until the scientific discoveries of the 17th century. His works contain valuable information on the history of medicine of the proto-scientific period. Each treatise is accompanied by a commentary, which presents a historical and medical analysis of Galen's main ideas. The works presented in the publication are a typical example of the connection between Galen’s general theoretical, natural philosophical views and his practical activities as a doctor; they demonstrate an excellent mastery of the empirical method and the skills of synthetic thinking based on the principles of rational medicine. The methodology proposed in the publication allows us to show the mutual influence of natural philosophy and medicine and comprehensively comprehend the historical significance of Galen’s works. The book is intended for scientists dealing with problems of history and philosophy of natural science.