Ijtihad has been derived from the root word Jahada. Ijtihad literally means striving or self-exertion. Ijtihad consists of intellectual exertion. Ijtihad is a very broad source of Islamic law and comes after the Quran and the Sunnah.
The Quran and the Sunnah were completed at the time of death of the Prophet. Ijtihad, however, continues and this is the source or methodology which gives Islamic law, its adaptability to new situations and capacity to tackle all new issues and problems. Propriety or justification of Ijtihad is measured by its harmony with the Quran and the Sunnah.
The sources of Islamic law other than the Quran and the Sunnah are essentially manifestations of Ijtihad. When clear rule is available in the text (Nass) of the Quran and the Sunnah, Ijtihad is not applicable. The findings of Ijtihad are essentially Zanni in character. The subject matter of Ijtihad is the practical rules of Shariah not covered by Nasus. Ijtihad is a duty of the scholars. If the issue is urgent, Ijtihad is compulsory on each competent scholar. (Fard al Ayn or Wajib al Ayn). If the issue is not urgent, it is a collective obligation (Fard al Kafai or Wajib al Kafai).
A scholar is supposed to avoid Taqlid (blind following of another scholar). Taqlid is permissible only for a layman. Ibn Hazm believes Taqlid is not permissible for any one.
Ijtihad is validated by the Quran and the Sunnah and the practice of the Sahabas. The Quran - 59:2; 9:122; 29:69; 4:59 have been quoted in support of Ijtihad. These Ayats are Zahir in nature ( i.e. liable to interpretation and as such only give rise to probability).
Several hadith are quoted in support of Ijtihad. Of them, is the Hadith in which the Prophet (SM) said that the Mujtahid will get two rewards if he is corrrect and one reward if he commits a mistake (Abu Dawood).
Requirements of Ijtihad have been laid down by some scholars. Nothing has been mentioned in this regard in the Quran and the Sunnah. Abul Hasan al Basri, laid down for the first time the qualifications of a Mujtahid in the 5th century Hijra which was later accepted by Gazali and Amidi. It is true that Ijtihad is the function of the competent schoars. The following are the requirements :
(a) mastery of the Arabic language, to minimise the possibility of misinterpreting Revelation on purely linguistic grounds;
(b) a profound knowledge of the Quran and Sunnah and the circumstances surrounding the revelation of each verse and hadith, together with a full knowledge of the Quranic and hadith commentaries, and a control of all the interpretative techniques discussed above;
(c) knowledge of the specialised disciplines of hadith, such as the assessment of narrators and of the matn [text];
(d) knowledge of the views of the Companions, Followers and the great imams, and of the positions and reasoning expounded in the textbooks of fiqh, combined with the knowledge of cases where a consensus (ijma) has been reached;
(e) knowledge of the science of juridical analogy (qiyas), its types and conditions;
(f) knowledge of ones own society and of public interest (maslahah);
(g) knowing the general objectives (maqasid) of the Shariah;
(h) a high degree of intelligence and personal piety, combined with the Islamic virtues of compassion, courtesy, and modesty.
Procedure of Ijtihad is that the Mujtahid must first look at the Quran and the Sunnah. Only if solution is not found there, he may resort to Ijtihad. Rules of Ijtihad by way of Qiyas, Istihsan, Istislah have already been discussed previously.
The majority hold that Ijtihad is liable to error. The minority hold that each of the several verdicts may be regarded as truth on their merit. (Shawkani, Irshad).
Mujtahids have been classified in various ways by some scholars according to their understanding.
The basic classification can be as follows :
1) Mujtahidun fil-Shar' - Mujtahid in issues of Shari'ah is the one who fulfilled in entirety all of the previously mentioned conditions as is attested to by the people of knowledge of his or her time. Such an individual is NOT permitted to follow a madh-hab.
Examples are: Ibrahim al-Nakha'i, Sufyan ath-Thawri, Al-Awza'i, al-Layth bin Sa'd, Ibn Rahawayah and others.
2) Mujtahidun fil-Madh-hab - Scholar who is qualified to differ with the opinions put forward within his madh-hab of study. Examples are Ibn 'Abdul Barr for the Malikis, Nawawi for Shafiis, Ibn Abdin for Hanafis, and Ibn Qudama for Hanbalis. These scholars are followers of the usul of their Madh-hab but have used their knowledge and understanding and judgment in deriving new verdicts within the madh-hab.
3) Mujtahidun fil-Masaail - Mujtahid in Particular issues. They remain within their madh-hab of study but are able to make ijtihad on certain aspects within the madh-hab that they are knowing of.
Some scholars were against Ijtihad after the first few centuries. This view has now been rejected. Shawkani said that this view is to be utterly rejected.