Doctrine of the Necessity for Juristic Reasoning

Adopted from the book : "The Faith of Shi'a Islam" by : Allamah Muhammad Ridha al-Muzaffar"

We believe that ijtihad in matters of religion is a sufficient necessity (wajib al-kifa'i) for all Muslims in the absence of the Imam. that is to say that should one of them become proficient in ijtihad and become a mujtahid it is enough for them to follow the mujtahid in all the branches of the religion.
All Muslims must strive to gain knowledge and to ascend to the position of mujtahid, or, if that is not possible. they must give all their encouragement to one of their number to attain this position. If no-one living holds the position of mujtahid, it is not permissible to follow a dead mujtahid.

Ijtahid is the examination of the sources of the shar'ah to reach knowledge of the commandments (al ahkam al-far'iyyah) which the prophet brought with him, and which do not change or alter with changes in time or situation according to the hadith:

What Muhammad (S.A) made halal will be able halal till the Day of Judgement, and what he made haram will be haram till the Day of Judgement.
These sources for the shari'ah are the Qur'an the sunnah (of the Prophet and the Imams), concensus (ijma) and reasoning ('aql), as have been mentioned in the texts of usul al-fiqh. Attaining the position of mujthaid requires many years of study and acquiring knowledge, and this is not obtained except by one who tries his utmost.