There is no disagreement among Muslim theologians that knowledge is necessary for the Imam. However, they have different views about the quantity and quality of his knowledge. Non-Shiite theologians consider ijtihadi (deductive) science [science obtained through effort and research] as sufficient which is subject to error. However, they do not consider it as necessary for the Imam to have a comprehensive knowledge of shari'a laws. Shiites on the contrary believe that Imam should have practical and comprehensive knowledge of shari'a laws; his knowledge should not be subject to error and he should be the most knowledgeable person of his time.
The scope of Imam's knowledge
There is not a consensus on Imam's knowledge among Shiite scholars. Nevertheless they are unanimous that Imam should know everything that is necessary for his imamate and mankind's happiness.
Ibn Meitham Bahrani, a Shiite theologian, says: "Imam should know everything required to perform his duties of imamate whether religious sciences or non-religious sciences; such as knowledge of shari'a laws, politics, manners, and settling disputes.[1]
Enumerating Imam's attributes, Muhaqqiq Tusi says: "The third attribute for Imam is his knowledge of every issue and all matters. That is to say, based on the judgment of the intellect, Imam should know everything about all his responsibilities of imamate; including knowledge of state affairs, political issues and what is expedient for people and what is not. He should also have knowledge of all the issues of shari'a and religion; because his ruling should comply with the injunctions of shari'a."[2]
Seyyed Mortaza, a great Shiite scholar, writes: "We seek refuge to God not to consider as necessary for Imam, sciences which are not related to his imamate and wilayat (guardianship)."[3]
Sources for Imam's knowledge
To find the answer to the present question, we must study the sources of Imam's knowledge and should find out as to where Imam's knowledge originate. Based on traditions narrated from holy Prophet and his household (S), the sources of Imam's knowledge are as follow:
1- Through the holy Prophet (S): Imam Ali (A.S) was brought up and reared at the Prophet's house. He was the holy Prophet's excellent disciple. Imam Ali (A.S) says: "The Messenger of Allah (S) taught me one thousand chapters (baab) of knowledge; each chapter consisted of yet another one thousand chapters."[4]
Someone asked Imam Sadiq (A.S) as to how he answered the questions. Imam Sadiq (A.S) said: "Any answer I give you is from holy Prophet (S)."[5]
2- Communication with angels: Communication with angels is not limited to the holy Prophet (S). The Quran says some individuals who were not prophet have communicated with angels such as Mary. One of the attributes mentioned for Imams in traditions is muhaddath, meaning that angels would speak with him.
Imam Sadiq (A.S) explains the meaning of muhaddath and says: "Imam hears the angel's sound but he does not see it."[6]
3- Special written sources which are accessible to Imams only; such as Lady Fatima's mus-haf (A.S) and Imam Ali's (A.S) Book.
4-Inspiration: Imam Baqir (A.S) says: "Imam Ali (A.S) would always act upon God's book and the holy Prophet's traditions (S) and whenever a new issue arose that was not mentioned in the Quran and tradition, God would inspire him with the truth."[7]
Imam Ali's (A.S) Knowledge
The fact that Imam Ali (A.S) is the most knowledgeable is crystal clear and confessed by everyone. Both Shiites and Sunnis have narrated that holy Prophet (S) has said: "I am the city of knowledge and Ali (A.S) is its gate".[8] Umar has said many times that "If it was not for Ali, Umar would have perished.[9]
Fakhr Razi, a great Sunni scholar, says about Imam Ali (A.S): "Ali (A.S) is pioneer in every science. Scholars in different fields of science are his students. In the field of Tafsir (exegesis) of the Quran, everyone was Abdullah bin Abbas' student and he was Imam Ali's (A.S) student. In the field of theology, there is no dispute that Imam Ali (A.S) initiated this science and that Ali (A.S) is the master of everyone in the world after holy Prophet (S)[10]. Therefore whatever has been narrated in Shiite and Sunni sources that Imam Ali (A.S) sent Meqdad to ask the holy Prophet (S) the ruling for mazi­ [11] is not in contrast to his expansive knowledge because, first, the holy Prophet (S) is one of the sources of Imam Ali's and other Imams' knowledge. Second, sending Meqdad to ask the holy Prophet (S) does not imply that Imam Ali (A.S) did not know the ruling; because there are other possibilities as well; for example by this act Imam Ali (A.S) would like everyone to know that his knowledge is from holy Prophet (S) and he did not say anything other than what he heard from Prophet (S).
There is another possibility here; Imam Ali (A.S) would try to teach people that during holy Prophet's time everyone should refer to him not others in order to maintain his sacredness and scientific authority.

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