The Translation itself. The plan of this work includes (a) a translation, in which the translator has tried to give a fair representation of the text in a not too ragged form but has failed to improve on the original. The works were written as science rather than literature and have many facts but no style. The translator has therefore aimed rather at representing these facts than at producing a piece of polite literature. (b) Notes are subjoined including, first the brief biographical data which every one wants first to orient himself by, secondly textual notes, and thirdly, occasional explanatory notes.