The foundations of ancient rational medicine were laid by Hippocrates, but the credit for rethinking medicine as a comprehensive system of protoscientific knowledge belongs to Galen. Before him, medicine had no unified system of thought on the basic principles of the structure of living things, and no systemic approach to clinical practice, based on the apodictic method. In this article, I examine Galen’s teachings on the digestive system (one of the key aspects of his system) in terms of the “commensurability” of the ideas of ancient medicine and modern science. An analysis of the information in Galen’s works indicates that his system of theory and practice is substantially commensurable with the principles of modern medicine: many aspects of modern theory and practice represent a development of Galen’s ideas.