Imam Ali (peace be upon him): In Tafsir al-Nu'mani, in the chapter on the Qur'an, it is reported that Amir al-Mu'minin (peace be upon him) said: Faith, disbelief, polytheism, and their increase and decrease are as follows: Faith in Allah is the highest of deeds in rank and holds the loftiest station; no station is nobler than it, and its share is greater than that of all others. It was said to him: "O Amir al-Mu'minin (peace be upon him)! Is faith speech and action, or is it speech alone?" He said: "Faith is affirmation in the heart, acknowledgement on the tongue, and action through the limbs. Faith is entirely action. Faith is sometimes complete, sometimes half-complete, sometimes deficient, and sometimes greater — such that this deficiency and increase is plainly seen. Allah the Exalted has assigned to each of the human limbs and organs a duty that has no relation to any other organ. One of them is the heart of the human being, by which he grasps matters and by which his understanding and awareness function, by which he solves problems and understands its good and evil. The heart is the commander and chief of the body, and every action which the other limbs and organs perform is carried out at the command of the heart, and commands and prohibitions in the body are executed through the heart ... No one believes in Allah unless he believes in the Messenger of Allah and His proofs (hujjahs) upon the earth. Allah says: 'Whoever obeys the Messenger has indeed obeyed Allah' (Nisa/80). It is not that Allah places among the limbs and organs of human beings a leader and imam that removes doubt and uncertainty from them and establishes certainty for them — and that imam is the heart of the human being — yet leaves this matter unattended in the case of the proofs (hujjahs) ... Then He clarified the position of those in authority (wulat al-amr), who are among the people of knowledge, and made their rank plain, and said in His Book: If they were to refer the matters of dispute to the Messenger and to those in authority among them, those with knowledge would extract the truth and make the reality plain (Nisa/83). From this it is clear that people are incapable of the ta'wil (interpretation) of the Book, and only those in authority can derive it, for they are the ones deeply rooted in knowledge (al-rasikhun fi al-'ilm), and they are the trustworthy authority regarding the ta'wil of the Qur'an. And He said in the Qur'an: 'None knows its interpretation except Allah and those firmly rooted in knowledge' (Al-i Imran/7). And in another place He said: They are clear signs residing in the breasts of the people of knowledge. And seeking knowledge and virtue is better than worship.