Gaius See COMMENTARIES ON PLATO, MIDDLE PLA -. TONIS. Galen (A.D. 129–c.215), physician and philosopher from Greek Asia Minor. He traveled extensively in the Greco-Roman world before settling in Rome and becoming court physician to Marcus Aurelius. His philosophical interests lay mainly in the philosophy of science (On the Therapeutic Method) and nature (On the Function of Parts), and in logic (Introduction to Logic, in which he develops a crude but pioneering treatment of the logic of relations). Galen espoused an extreme form of directed teleology in natural explanation, and sought to develop a syncretist picture of cause and explanation drawing on Plato, Aristotle, the Stoics, and preceding medical writers, notably Hippocrates, whose views he attempted to harmonize with those of Plato (On the Doctrines of Hippocrates and Plato). He wrote on philosophical psychology (On the Passions and Errors of the Soul); his materialist account of mind (Mental Characteristics Are Caused by Bodily Conditions) is notable for its caution in approaching issues (such as the actual nature of the substance of the soul and the age and structure of the universe) that he regarded as undecidable. In physiology, he adopted a version of the four-humor theory, that health consists in an appropriate balance of four basic bodily constituents (blood, black bile, yellow bile, and phlegm), and disease in a corresponding imbalance (a view owed ultimately to Hippocrates). He sided with the rationalist physicians against the empiricists, holding that it was possible to elaborate and to support theories concerning the fundamentals of the human body; but he stressed the importance of observation and experiment, in particular in anatomy (he discovered the function of the recurrent laryngeal nerve by dissection and ligation). Via the Arabic tradition, Galen became the most influential doctor of the ancient world; his influence persisted, in spite of the discoveries of the seventeenth century, until the end of the nineteenth century. He also wrote extensively on semantics, but these texts are lost. R.J.H.