Galen developed his Medical knowledge through periods studying at the Asclepion at Pergamum, through a short spell working in Alexandria and as a doctor to the Gladiators in Rome. These experiences allowed Galen to develop an understanding of anatomy, and led him to have a firm believe that clinical observation as professed by Hippocrates, was a necessity in curing ailments of all kinds. Galen rose to prominence following his appointment as the surgeon to the Emperors son, Commodus. This allowed him to study and teach medicine: which led to his development of ideas and his establishment of new laws of medicine.
Galen studied the bodies of animals to support his research. particularly he used Barbary Apes which are very similar in terms of anatomy to Humans. This type of research, along with the dissection of human remains that he conducted in Alexandria, led to the development of his theory on the Human Body’s physiological system. This was a remarkable, if slightly incorrect, development which would allow doctors and physicians to clearly understand the effects of the treatments given.
Galen’s work was painstaking. His writings always dealt with possible objections and criticisms of theories and he regularly reviewed practices. The depth of his writings and the support of the authorities (including the religious authorities) led to his belief in clinical observation and diagnosis becoming the standard practice for doctors in Europe over the course of the next thousand years.