Whilst the greatest effort has been made to ensure the quality of this text, due to the historical nature of this content, in some rare cases there may be minor issues with legibility. In this new edition of my translation of the Organon I have completely revised the text, in order to make it a still more exact reproduction of the original. In the Appendix I have given all the more important variations of the previous editions. I have also indicated the corresponding views as set forth in the Essay on a New Principle and the Medicine of Experience, both of which essays may be regarded as the precursor of the Organon. I have added Hahnemann’s later opinions on several subjects treated of in this work. In the growth of such a complex thing as a new system of medicine, it was inevitable that there should be considerable alterations and improvements effected in the course of forty-eight years, the time occupied by Hahnemann in the elaboration of his novel doctrine and practice. His first idea of the homoeopathic rule of practice occurred to him while translating Cullen’s Materia Medica in 1790. The Essay on a New Principle in which he propounded the homoeopathic therapeutic rule, as yet believed by him to be of only partial application, viz. to some chronic diseases, was published in 1796. Nine years after this, viz. in 1805, in the Medicine of Experience, he enunciated the rule with no such limitations of its applicability. This essay contains much of what we find in the first and later editions of the Organon. The first edition of this latter work appeared in 1810. The second edition, differing very considerably from the first, was published in 1819. The third edition, which hardly differed at all from the previous one, appeared in 1824.