Book excerpt: Allah preserved the Sunnah by enabling the Companions and those after them to memorise, write down and pass on the statements of the Messenger Muhammad and the descriptions of his Way, as well as to continue the blessings of practising the Sunnah. Later, as the purity of the knowledge of the Sunnah became threatened, Allah caused the Muslim nation to produce outstanding individuals of incredible memory-skills and analytical expertise, who journeyed tirelessly to collect hundreds of thousands of narrations and distinguish the true words of precious wisdom of their Messenger from those corrupted by weak memories, from forgeries by unscrupulous liars, and from the statements of the enormous number of 'ulama', the Companions and those who followed their way, who had taught in various centres of learning and helped to transmit the legacy of Muhammad - all of this achieved through precise attention to the words narrated and detailed familiarity with the biographies of the thousands of reporters of Hadith. The methodology of the expert scholars of Hadith in assessing such narrations and sorting out the genuine from the mistaken/fabricated etc., forms the subject-matter of a wealth of material left to us by the muhaddithun (scholars of Hadith, "traditionists"). This short treatise is a humble effort to introduce this extremely wide subject to English readers. A useful supplement is included for the first time to the original treatise further expounding on branches in the Science of Hadith.