The third edition of this book has been out of stock for several years. In this fourth edition, I have only made some minor revision. Only a few paragraphs have been added to Chapter Two. Hopefully, some minor modifications in respect of wording in several places have made this book a little more readable. I have also taken this opportunity to rectify many typological errors and stylistic inconsistencies. I am grateful to my Bangladeshi pupil, Venerable Jnanananda Tangchangya (Jnan Nanda), for putting the whole book into the InDesign format, and for looking after the proof-reading and other aspects of this publications. I must also thank my pupil, Sree Dharma Rakkhit (Shimul Barua) and Andrew Lau Ting Kwong for checking through the whole Index, and also my PhD students, Gao Mingyuan and Tan Lingfeng for helping to detect some of the typos.