Islam and the Issue of Jurisprudency (Ijtihad)
The right of ijtihad did not last for long among the Sunnis. Perhaps the cause of this was the difficulty which occurred in practice: for if such a right were to continue [for any great length of time], especially if taawwul and the precedence of something over the texts were to be permitted, and everyone were permitted to change or interpret according to his own opinion, nothing would remain of the way of Islam (din alislam). Perhaps it is for this reason that the right of independent ijtihad was gradually withdrawn, and the view of the Sunni ulama became that they instructed people to practice taqlid of only the four mujtahids, the four famous Imams - Abu Hanifa [d.150/767], alShafii; [d.204/820], Malik b. Anas [d.179/795] and Ahmad b. Hanbal [d.241/855] - and forbade people to follow anyone apart from these four persons. This measure was first taken in
Ijtihad permitted by the shari'a.
The word ijtihad was used until the fifth hijri century with this particular meaning, i.e., with the meaning of qiyas and ijtihad alra'y, a kind of ijtihad which is prohibited in the eyes of the Shia. Up to that time, the Shii ulama included a chapter on ijtihad in their books only because they wanted to refute it, to emphasize that it was null and void, and to proscribe it, as did the Shaykh alTusi in some of his works. But the meaning of this word gradually extended beyond this specific meaning, and the Sunni ulama themselves began not to use 'ijtihad' in the specific sense of ijtihad alra'y, [as a source] which was on the same level as the Book and the Sunna. [Such a shift in the meaning of the word can be seen with] Ibn Hajib  in his "Mukhtasar alusul", on which Adud alDin alIji wrote a commentary known as alAdudi, and which has been till recently, and maybe still is, the authoritatively approved book on [Sunni] usul, and before him with alGhazali  in his famous work "alMustasfa". It then became used rather in the unqualified sense of effort or exertion to arrive at the rulings of the sharia, and was defined as "the maximum employment of effort and exertion in deducing the rulings of the sharia from the valid proofs (adilla, sing. dalil, see below ). However, it is another matter to decide what the valid proofs of the sharia are: whether qiyas, istihsan, and so forth, are among them or not.
From this time onwards, the Shii ulama also adopted this word because they accepted this [general] meaning. This kind of ijtihad was a kind approved by the sharia. Although the word had originally been one to be avoided among the Shia, after its meaning and the concept it denoted had undergone this change, their ulama, discarded their prejudice and subsequently had no reservations about using it. It seems that in many instances the Shii ulama, were careful to consider unity of method and conformity among Muslims as a whole. For example, the Sunnis came to recognize ijma (consensus of opinion among the ulama) as a proof leading to certainty, and, in practice, they also held it to be fundamental and substantive (mawdui) just like qiyas, whereas the Shia did not accept it. However, to protect the unity of method, they gave the name ijma to a principle which they did accept  . The Sunnis said that the valid proofs were four in number: the Book, the Sunna, ijma and ijtihad (qiyas); the Shia said the valid proofs were four: the Book, the Sunna, ijma and aql (reason). They merely substituted aql for qiyas.
At any rate, 'ijtihad' gradually found a wider meaning, i.e., the employment of careful consideration and reasoning in reaching an understanding of the valid proofs of the sharia. This, of course needs a series of sciences as a suitable preliminary basis on which to develop the ability to consider and reason correctly and systematically. The ulama of Islam gradually realized that the deduction and derivation of the precepts from the combined valid proofs of the sharia necessitated [the learning] of a series of preparatory sciences and studies such as the sciences of literature, logic, the Qur'anic sciences and tafsir (Qur'anic exegesis), the science of hadith and the narrators of hadith (rijal alhadith), the science of the methodology of usul alfiqh, and even a knowledge of the fiqh of the other sects of Islam. A mujtahid was someone who was a master of all these sciences.
I think it extremely likely, though I cannot state this categorically, that the first person among the Shia to use the words ijtihad and mujtahid [positively] was the Allama alHilli. In his work " Tahdhib alusul'', he puts the chapter on ijtihad after the chapter on qiyas, and there he uses the word in the same sense in which it is used today.
[We can therefore say that] the ijtihad which is forbidden and rejected in the eyes of the Shia is ra' y and qiyas, which were originally called ijtihad, whether this is counted as a source of the sharia and as an independent basis for legislation, or taken as a means for deriving and deducing true precepts; whereas the ijtihad which they deem correct according to the sharia is that which means effort and exertion based on expert technical knowledge.
In answer to the question: what is the meaning, the use and the place of ijtihad in Islam, it can thus be said that it is ijtihad in the meaning that it is used today, i.e., competence and expert technical knowledge. It is obvious that someone who wants to refer to the Qur'an and hadith must know how to explain the meaning of the Qur'an, he must know the meaning of the verses, which verses abrogate which verses, which ones have clear meanings and which ones ambiguous meanings  - and he must be able to distinguish which hadith is valid and authoritative and which not. In addition, he must understand, on the basis of correct rational principles, incompatibilities between hadiths to the extent that it is possible for him to resolve them, and he must be able to distinguish the cases in which the ulama of the Shia sect have consensus (ijma). In the verses of the Qur'an themselves, and similarly in the hadith, a series of general principles [for verification and interpretation] are laid down, and the use and exercise of these principles need training and practice, just as in the case of all other basic principles in every science. Like the skilled technician who knows which material to choose from all the materials available to him, the mujtahid must have proficiency and ability. In hadith, especially, there is a great deal of fabrication, the true and the false are mixed together; the expert must have the power to distinguish between them. In short, he must have enough preliminary knowledge so that he can exercise competence, authority and technical expertise.
The appearance of the Akhbaris in Shi'i Islam
Here we must mention an important and perilous current which first appeared around four centuries ago in the Shii world over the question of ijtihad - Akhbarism. If a group of the ulama had not been forthright and challenged it, and had not taken a stand against this current and destroyed it, there is no knowing in what position we should be today.