Traditionally, one of the sources of understanding Islam is consensus (ijmā‘). According to Shi‘a methodology of thought, the consensus of all people or a group of them such as the scholars by itself is not sufficient as a proof (hujjah); just as one person may make mistake, two, three, or thousands, or even all of them may do so. However, whenever there exists an agreement among all Muslims or Muslim scholars in a way whereby the agreement unveils the Sunnah, it can serve as a proof, as an instrument to uncover the will of God. For example, when we find that every Muslim in the time of the Prophet said his prayer in a certain way we realize that the Prophet had instructed them to do so; otherwise there would be no factor to unify their action. It is not possible to imagine that they had all acted blindly and without instruction, or that they all made mistakes and the Prophet did not correct them.
Thus, for the Shi‘a consensus in itself is not a proof. It only works when it leads to the discovery of Sunnah. Accordingly, if Muslims today agree on a given subject, while a scholar has doubt about the Islamic judgement on that subject, he methodologically cannot say that because everybody says so, I also say the same. There have been many cases in the history where all human beings believed in the same way and later they found out that they were wrong, e.g. the earth being flat. It is only the Qur’an and the Sunnah that are unquestionably true and immune from any error or mistake.
This approach grants a type of dynamism to Shi‘i thought, so that every generation of scholars and even any single scholar is able and indeed is required to refer directly to the Qur’an and Sunnah and conduct his own original ijtihād, that is his investigation and independent judgement. Ijtihād has never been banned or closed in the Shi‘a world. The Shi‘a believe that the view of no jurist, however high his position, is immune from scientific questioning or challenge. Of course, as in any other discipline, every religious scholar needs to consult and examine carefully the works of his predecessors.