Among the sciences which Ima`m al-Ba`qir, peace be on him, split open was the science of fundamentals (‘Ilm al-Usu`l). It is among the greatest Islamic sciences after jurisprudence. That is because Ijtiha`d (concluding religious opinions) depends on it. The mujtahid (one who is capable of concluding religious opinions) is unable to have the talent of Ijtiha`d without studying the chapters of this science.(Kifayat al-Usul)
The researchers and the scholars have unanimously agreed that Ima`m Abu` Ja’far (al-Ba`qir), peace be on him, was the first to establish this science.
Sayyid Hasan al-Sadr said: “Ima`m Abu` Ja’far, Mohammed b. ‘Ali al-Ba`qir, peace be on him, was the first to open the chapters of this science and to split open its problems. Then his son Abu` ‘Abd Allah, peace be on him, came after him to continue it. Then they dictated its rules and problems to a group of their students. Then the later generations regulated its chapters in books such as Usu`l Al al-Rasu`l, al-Fusu`l al-Muhima fi Usu`l al-A’imma, and al-Usu`l al-Asilah. The reliable narrators reported all these chapters on the authority of the members of the House (ahl al-Bayt), peace be on them.(Al-Shi’a wa Funun al-Islam, p. 95)
The following are some of the fundamentals which Ima`m Abu` Ja’far al-Ba`qir, peace be on him, established or reported on the authority of his pure grandfathers, and to which Moslem jurists resort to give religious opinions when they find no tradition concerning the legal precept. Though most of them are jurisprudential rules, the scholars of these fundamentals have mentioned them in detail in the science of fundamentals (‘Ilm al-Usu`l). However, we will mention them here.
Presumption of Continuity (Istisha`b)
It is one of the four fundamentals to which the doubter resorts during an action. The reason for his doubt is either that the text (tradition) does not exist or the text is general or the texts (traditions) are contradictory. The texts are invalid when they are equal, namely they are not superior to each other.
Presumption of continuity happens when conviction precedes doubt. The Ima`m, peace be on him, said that presumption of continuity was evidence. That was when he was asked about the doubts in the prayer. Some jurisprudence encyclopedias have mentioned these problems. Among them is Wasa`’il al-Shi’a.
The Rule of Passing (Qa’idat al-Tajawiz)
This rule means that the doubt occurs in the subsequent act.(Hada’iq al-Usul, vol.2, p. 547)
For example, the person may doubt his recitation after he has bowed down. Concerning this rule, many traditions were narrated on the authority of Ima`m al-Ba`qir, peace be on him, and his son Ima`m al-Sa`diq, peace be on him.
They denote that the person should pay no attention to this doubt and to go on with his praying.
The Rule of Finishing (Qa“’idat al-Fara`gh)
This rule means that the act is correct at the time of doubt.(Hada’iq al-Usul, vol.2, p. 547)
This rule was reported on the authority of Mohammed b. Moslem on the authority of Ima`m Abu` Ja’far (al-Ba`qir), peace be on him, who said: “If you doubt all that which you have already completed, then let it as it is.(Mustamsak al-‘Irwat al-Withqa, vol. 7, p. 350)
A true tradition which Mohammed b. Moslem reported on the authority of al-Ba`qir, peace be on him, also proved that. In this tradition it has been mentioned: “If doubt occurs after the time of the prayer has passed, one should ignore the doubt and not repeat (the prayer)(Mustamsak al-‘Irwat al-Withqa, vol. 7, p. 349)
In the light of this reliable true tradition, the Ima`mi Shi’ites have given a religious opinion. This religious opinion says that one should pay no attention to the doubt that occurs in the acts of the prayer which he has completed.
The Rule of Removing Harm
Among the important rules in Islamic law is the rule of removing harm. Shaykh al-Ansa`ri said: “It means removing the legal precept that leads to harm. “The jurists have mentioned many legal precepts regarding this rule. Ima`m Abu` Ja’far (al-Ba`qir), peace be on him, underlined this rule when he, peace be on him, said to Zara`ra: “Samra b. Jundub had a date-palm in the house of a man from the Ansa`r. The man’s house was facing the gate of the garden. Samra entered the man’s house without permission. Thus, the man spoke to Samra to ask his permission before entering. However, Samra refused that.
So, the man came to the Prophet, may Allah bless him and his family, and told him about that. Then Allah’s Apostle, may Allah bless him and his family, sent for Samra and ordered him to ask the man’s permission. However, Samra refused that. So, the Prophet, may Allah bless him and his family, asked him to sell the date-palm at a good price. Still, Samra refused to sell it.
Then Allah’s Apostle, may Allah bless him and his family, said to him: ‘You shall have a date-palm in the garden.’ Nevertheless, Samra refused that. So, Allah’s Apostle, may Allah bless him and his family, said to the (Ansa`ri) man: ‘Go and uproot the date-palm and throw it at him for there is no harm.