The Preface to the first edition reproduced in this book was written by the author himself. It explains clearly the approach of the author to the subject as well as the rationale and methodology he has followed in the treatment of the most important body of thought contained in the major Upanishads. As early as when he wrote out this book on the thirteen Upanishads which are considered to be the earliest and the most seminal, he was already considering the proposition of writing on the minor and later Upanishads also.