The article is devoted to the analysis of the evident scientific method of Galen, which establishes the necessity of correct diagnosis of diseases, determination of true symptoms and causes of diseases, which results in the choice of the exact method of treatment. The article focuses on how Galen seeks to achieve reliable knowledge based on an undeniable logical necessity. Logical reliability is contrasted with “dialectical”, that is, probabilistic judgments, often leading to the opposite of what was originally asserted in them. (...) Probabilistic judgments were characteristic of the Stoic philosophical school, with which Galen hotly argues, asserting his understanding of truth. Truth, in his opinion, is achieved through facts based on true premises. The criterion of truth for Galen is the study of the device of a living organism, and the logical conclusions depend on the accuracy of knowledge of human anatomy. Thus, the nature of the proof judgments of Galen depends on their practical content and not on the formal and logical structure offered by the Stoics when studying the functions of the human body, which ultimately leads to incorrect methods of treatment. (shrink)