Aelius Galenus or Claudius Galenus (September AD 129 – 199 or 217) better known as Galen of Pergamon (modern-day Bergama, Turkey) was a prominent Roman (of Greek ethnicity) physician, surgeon and philosopher.  Arguably the most accomplished of all medical researchers of antiquity, Galen contributed greatly to the understanding of numerous scientific disciplines including anatomy, physiology, pathology, pharmacology and neurology, as well as philosophy and logic.

He travelled extensively, exposing himself to a wide variety of medical theories and discoveries before settling in Rome, where he served prominent members of Roman society and eventually was given the position of personal physician to several emperors.

His theories dominated and influenced Western medical science for nearly two millennia. His anatomical reports, based mainly on dissection of monkeys and pigs, remained uncontested until 1543, when printed descriptions and illustrations of human dissections were published. Galen's theory of the physiology of the circulatory system endured until 1628. Medical students continued to study Galen's writings until well into the 19th century. Galen conducted many nerve ligation experiments that supported the theory, which is still believed today, that the brain controls all the motions of the muscles by means of the cranial and peripheral nervous systems.

Galen was highly interested in the importance of combining philosophical thought with medical practice. He refused to be placed into one particular school of thought, instead taking aspects from each group and combining them with his original thoughts to form his own unique approach to medicine. He was a proponent of medicine as a highly interdisciplinary field that was best practiced by utilizing theory, observation, and experimentation in conjunction to yield the most complete results.