If you count area covered (the Roman Empire and beyond), proportion of the population (just about everyone) and time in the charts (well over fifteen hundred years) Galen is the most influential doctor there has ever been.
Hippocrates his closest rival lived 500 years before and did not have Galen's breadth. This book is an easy introduction to an astounding man.
Galen used a holistic approach based on lifestyle and mood. Over the last three hundred years, his theories fell out of fashion, to be replaced by the modern mechanistic medical model.
This is a simple introduction to Galen's views of medicine, and the four humours (moods). It is only early in the 21st century are elements of his work being revived. This little book, childlike in its simplicity provides an easy introduction to his work.
A useful start for people who are interested in Galen but who have no idea where to begin