for elevation of the importance of hadith in Islamic law, Imam Al-Shafi‘i, "In most cases the truthfulness or lack of truthfulness of a tradition can only be known through the truthfulness or lack of truthfulness of the transmitter, except in a few special cases when he relates what cannot possibly be the case, or what is contradicted by better-authenticated information." The first people who received hadith were Muhammad's "Companions" (Sahaba), who are believed to have understood and preserved it. They conveyed it to those after them as they were commanded; then the generation following them, the "Followers" (Tabi‘un), received it and then conveyed it to those after them, and so on. Thus, the Companion would say, “I heard the Prophet say such and such.” The Follower would say, “I heard a Companion say, ‘I heard the Prophet say’” The one after the Follower would say, “I heard a Follower say, ‘I heard a Companion say, ‘I heard the Prophet say’” and so on. Criteria to be a ṣaḥīḥ hadith To be 'ṣaḥīḥ ("sound") hadith, an isolated hadith (Mutawatir hadith were exempt from these tests) "must pass five tests": "continuity of transmission"; ʿadāla of transmitters, i.e. transmitters must be of good character; "accuracy (ḍabṭ) of the process of transmission, i.e. narrators must not be prone to carelessness or known to have poor memories"; absence of "irregularities" (shadhūdh), i.e. hadith must not contradict a "more reliable source"; "absence of corrupting defects(ʿilla qādiḥa), i.e. inaccuracies in reporting the actual chain of transmission." Biographical evaluation An important discipline within hadith studies is biographical evaluation, the study of transmitters of hadith, ʿilm al-rijāl, (literally "science of men") mentioned above. These are the narrators who make up the sanad. Ilm ar-rijal is based on certain verses of the Quran. Transmitters are studied and rated for their "general capacity" (ḍābit; itqān) and their moral character (ʿadāla). General capacity is measured by qualities such as memory, linguistic ability. Transmitters that have good memories and linguistic ability "might be considered competent (ḍābit)". ʿadāla transmitters must be "adult Muslims, fully in control of their mental faculties, aware of their moral responsibilities, free from guilt for major sins, and not prone to minor sins". Examples of ratings of transmitters include "trustworthy" or thiqa for ones that possess both ʿadāla and ḍābit. Transmitters that are ʿadāla but show signs of carelessness are rated honest or ṣudūq. The result of this study were