Galen was a prominent Greek physician and philosopher who lived during the Roman Empire, around 129 to 216 AD. He is best known for his contributions to medicine, particularly in anatomy and physiology. Galen's extensive writings influenced medical practice for centuries, emphasizing the importance of observation and experimentation. His work laid the foundation for many medical concepts, including the theory of the four humors, which suggested that health depended on the balance of bodily fluids. Galen's influence extended beyond his lifetime, impacting later figures such as Avicenna and shaping the development of Western medicine.