The Organon of rational therapeutics is a book written by Samuel Hahnemann that outlines the tenets of Homeopathy.  It was published in 6th editions, the last in the early part of the 20th century.  It is considered the basic text of Homeopathy. 

 

The online version of the 5th and 6th editions of the Organon can be found at Will Taylor's Homeopathic Web Site