The sources of ijtihad according to Shi'ah and the Ahl al‑Sunnah, put together, are: the Book, the Sunnah, ijma (consensus), aql (reason), qiyas (analogy), istihsan, masalih mursalah, istislah, sadd al‑dhara'i, fath al‑dhara'i; madhhab al‑sahabi, shariat al‑salaf, urf, istidlal, and so on. In this series of articles we shall try to discuss and study these topics in detail from the Shii and the Sunni points of view. We expect writers and scholars who have specialty in this field to contribute to the soundness and validity of these studies through constructive criticism relevant to the subject studied here. Any constructive criticism is very useful for correcting errors and removing shortcomings, and effective in eliminating ambiguities and omissions. Besides rendering vital service to scholarship, it is beneficial to the author, who is forced to be more careful in his statements and precise in his research. The intellectual history of Islam is indicative of the debt that the development and expansion of legal and other scientific studies owe to diverse viewpoints and competent criticism. This commendable and beneficial tradition has been in vogue in its most desirable form throughout the seven epochs of ijtihad among all the scholars and mujtahidun, and especially in the sixth and the seventh periods, whose fore‑runners were Wahid Behbahani and the great al Shaykh al‑'Ansari respectively, this tradition reached its zenith. Also, the very force of ijtihad, from the era of tashri (legislation) to the present (as discussed in detail in the article on the epochs of ijtihad), as a legitimate activity is derived from criticism and debate. Because, in Islamic fiqh, ijtihad has had the meaning of a free and independent effort of the mujtahidun, undertaken for the purpose of advancement and expansion of the Islamic sciences. This development was not possible except through freedom of scholarly research, free expression of different views and clash between views of the mujtahidun, scholars and thinkers. By practising ijtihad, a mujtahid deduces the ahkam (laws) of the Shari'ah for issues and problems regarding which there is no specific express text (nass), by relying on legal sources and principles and by benefiting from the ideas of other mujtahidun. On account of this, it may be said that ijtihad plays the role of an evolutionary and dynamic force in legal studies which provides solutions to contingent issues of life and fulfils the needs of changing times and the requirements of new phenomena of human civilization. Therefore, Islamic fiqh does not suffer with inertia, stagnation and passivity vis‑a‑vis the demands of the times, of life and its manifestations; it also advances along with them. It is for this reason that it has been said that ijtihad has been throughout Islamic history the force which has been constantly developing and expanding the boundaries of fiqh with respect to its applications, while maintaining the stability of legislation. Thus, ijtihad is essential for Islamic fiqh, without which it cannot be dynamic and progressive. On the basis of this, closing of the gates of ijtihad is contrary to the perennial mission of Islam in all ages, and, most certainly, alien elements have played an active role in promoting this ominous venture. For the enemies of Islam, it has been, and still is, the best weapon to strike Islamic law with, and the most effective instrument for eliminating it from scientific, cultural, economic, social and political arenas. Because, closing of the gates of ijtihad renders fiqh ineffective and incapable of providing answers to emergent and contingent issues of life The present deficiencies in the world of Islam, the failure to confront the contingent issues in a proper way, the issuing of baseless and irrational fatawa, the improper attitude towards new ideas ‑ all these are consequencies of the closing of the gates of ijtihad by the Ahl al-Sunnah. It has been instrumental in allowing dubious hands, with the aid of taghuti governments, to instil unhealthy ideas into the people's minds and to insinuate the feeling that Islamic fiqh cannot fulfil the demands of the present age and the modern civilization.

The Term Ijtihad: ‘Ijtihad', according to the lexicographers, is derived from juhd', which means employment of effort or endeavour in performing a certain activity.

Ibn al‑'Athir defines ijtihad' as the effort and endeavour undertaken for attaining some objective. He further remarks that the word occurs in many ahdith. 'Juhd' means employing ones complete strength, and jahd' means hardship and difficulty.  Ibn Manzur al‑Misri says: Jahd and juhd mean power and strength. He adds that it is said that whereas jahd means hardship and difficulty, juhd gives the sense of power and strength.  Later on he quotes al‑Farra' to the effect that in the verse of the Quran  jahd is used in the sense of power and strength. In the same work, he states that ijtihad and tajahud mean exertion of power and strength. In the hadith narrated from Mu'adh the phrase used in the sense of effort and endeavour to achieve some purpose. Said al‑Khuri says: Ijtihad means undertaking effort and endeavour in performing some task. For instance it is He exerted himself to carry the stone', but nobody says: (Khardalah=mustard seed). He further adds that jahd (verbal of jahada) is used in the sense of strength.