We believe that Allah has endowed us with the faculty of the intellect ('aql), and that He has ordered us to ponder over His Creation, nothing with care the signs of His power and His Glory throughout the entire universe as well as ourselves. It is stated in the Qur'an: 

We shall show them Our signs on the horizons and in themselves, till it is clear to them that it is the Truth. (41;53) 

Allah has shown His disapproved of those who blindly follow the ways of those who were before them: 

They say: "No, but we will follow such things as we found our father doing." What! And if their fathers had no understanding of anything. (2;170) 

and he has shown his dislike for those who follow nothing but their own personal whims: 

They follow naught but an opinion. 

Indeed, our intellect forces us to reflect upon Creation so as to know the Creator of the universe, just as it makes it necessary for us to examine the claims of someone to prophethood and to consider the truth of his miracles. It is not correct to accept the ideas of someone without criticism, even if that person has the gift of great knowledge or holds an esteemed position. 

The reason that the Quran has urged us to reflect upon Creation and 

to study the natural world and acquire knowledge is so that it may confirm in us mankind's instinctual freedom of thought upon which all sages are in agreement, in other the power of human understanding and thinking may be so enlightened that it may fulfill the function for which it was created. 

Thus it is wrong for a man to neglect the fundamentals of his faith and to put confidence in what his instructors or other persons have said to him. On the Contrary, it is necessary for him, on account of his natural intellegence as confirmed in the Qur'an, to search for, examine and come to clear understanding of the fundamentals of his religion (usul ad-din), Prophecy (nubuwwah), the Imamate (imamah) and the Resurrection (ma'ad). One who follows the dictates of his forbears or any other person in these matters has, in truth, committed an error and has wandered from the straight path, and there can be no excuse for such an action. 

In short, there are two points: 

(a) that it is necessary to examine and understand the fundamentals our beliefs and not to follow what anyone else might say. 

(b) that this is a requirement that stems from reason, and not one that we must accept the religion (i.e. such knowledge is not only to be gained from religious texts, even if, as we have seen, it is confirmed there, but it is also to be obtained through the exercise of the intellect). The meaning of a requirement that stems from reason is that reason enjoins us to acquire such knowledge and to examine these matters for ourselves.

By the branches of the religion we mean those religious laws which govern our actions (shari'ah), but concerning which not everyone is 

required to exercise juristic reasoning (ijtihad). However, it is the duty of everyone to take one of the three following ways in regard to them: 

(a) he should exert himself in study until he becomes a mujtahid, 

(b)he should exercise juristic precaution (ihtiyat) if he is able to, 

(c) he should follow one who is a recognized mujtahid, who must be a man of wisdom and justice, "who keeps himself from sin, not following the dictates of his own desires, but obeying the commands of Allah" 

It follows that one who is neither a mujtahid, nor exercise ihtiyat, nor follows a mujtahid, does all the actions of his din in vain, and that neither his prayer nor his fasting will be accepted by Allah, even though he has carried out his duties in these matters for the whole of his life;unless he begins to follow a mujtahid, in which case, those of his actions prior to his following the mujtahid which were done for the sake of Allah will be accepted.

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.

We believe that a fully qualified mujtahid is a representative of the Imam, in the case of the latter's absence. Thus he is an authority over Muslims and he performs the functions of the Imam as regard judgement and administration among the people. Because of this, Imam Ja'far as-Sadiq said: 

To deny the authority of mujtahid is to deny the authority Imam, and to deny the authority of the Imam is to make an objection to the authority of Allah, and this is tantamount to polytheism (shirk). 

Therefore the qualified mujtahid is not one who issues fatwas, but he also has general authority over Muslims who must consult him if they require judgement, this being obtainable only from him. It is correspondingly wrong for anyone to give judgement except him or one who is appointed by him, as no-one can pass sentence without his permission. Also, all that which belongs to the Imam should be given to the mujtahid. 

Such authority has been bestowed upon the qualified mujtahid by the Imam so that he may represent him in his absence ; hence he is known as the representative of the Imam (na' ib al-imam).