According to Islam, Ijtihad[i] is a necessity and the only way through which Islam, the real religion, can survive over the history and ages. Shiite jurisprudents and religious authorities differ on the details of secondary issues, not on the fundamentals of faith, tenets of religion, generalities, universal principles and also the conspicuous secondary issues such as prayers, fasting, hajj etc. They are unanimous about many of the sub-issues and laws either. Due to the significance of Ijtihad which rests in the continuity and permanence of the religion, Islam has overlooked some minor discrepancies which may sometimes arise among jurisprudents.
[i] Ijtihad is a technical term of Islamic law that describes the process of making a legal decision by independent interpretation of the legal sources, the Qur'an and the Sunnah.
We clarify two points as an answer to the above question:
First: Reason for discrepancies in religious scholars’ verdicts
As for the reason for discrepancies existing in religious scholars’ verdicts, it must be noted, firstly, that the Arabic word “Ijtihad” means to endeavor, strive, put oneself out, work hard. In Islamic legal terminology it is a technical term of Islamic law that describes the process of making a legal decision by independent interpretation of the legal sources, the Qur'an and the Sunnah.
Secondly, although a jurist’s verdict is principally based on certain sources (such as the Quran, Sunnah, reason and consensus), in order for him to deduce and infer Islamic laws there is a need for other sciences such as Arabic literature, knowledge of the colloquial language at the time of the Infallibles (a.s.), knowledge of the conceptual and judgmental principles of the sources of Ijtihad, logic, principles of jurisprudence, biography, Quran, hadith and so forth. Not only every jurisprudent should be well acquainted with all these sciences but he should also use a special groundwork so that he may be able to apply these sciences towards benefiting from legal sources. For instance, when it comes to using traditions and narrations it is likely that a Mujtahid who has adopted certain principles in Ilm-e Rijal (science of biography) may consider a tradition to be invalid because of its unreliable chain of transmission. When such a tradition is discarded and where there is no other tradition, this jurisprudent cannot give a fatwa on that issue whereas other jurisprudents who consider the same tradition to be reliable and authentic give fatwa with reference to it. There is no doubt that religious authorities and dignified jurisprudents do not enjoy the same level of analytical power or the competence to deduce Islamic laws. Therefore, each and every Mujtahid may end up having a different understanding of verses and traditions. Likewise, it is likely of a Mujtahid to consider one thing as the subject and manifestation of a ruling whereas another Mujtahid may not hold this view.
Having said that, the differences existing in the verdicts of grand and dignified religious scholars are natural.
Second: Discrepancies among religious scholars is in no way in contradiction with the oneness of Islam. To clarify this, we will have to explain a few points:
A) Islam has many rulings and laws pertaining to man’s different needs (materialistic and spiritual, individual and social, political and economical, etc.). These rulings have been gathered and compiled into various collections and volumes, making up “Fiqh”. In reality, fiqh is the true way of being Allah’s servant, the correct and humane method of social relationships, the peak of all systems of living that administer different aspects of everyday life, and as Imam Khomeini puts it, fiqh is the true and complete guideline for the administration of the individual and society, from the cradle to the grave.
Because of the very high importance of fiqh and Islamic practical rulings, the imams, the great guardians of Islam, have urged their followers to learn fiqh and have scolded those who have been careless and lazy regarding such an important matter.
Imam Baqir (a.s.) states: “If a Shia youth is brought to me who doesn’t know fiqh, I will discipline him!” In Islam, there are wajibs (obligatory acts) and harams (forbidden acts) that the All-Wise Lord has legislated for man’s prosperity in this world and the next and if one doesn’t act according to these Islamic laws, not only will he not prosper, but he also won’t be immune to chastisement in the hereafter. As was mentioned above, in order for one to get acquainted with one’s practical laws, many prerequisites are necessary, such as: the correct understanding of quranic verses and hadiths, being able to distinguish between authentic and fake hadiths, how to combine and gather hadiths, and tens of other subjects that will take years and years of hard work and effort to learn and master.
In such a condition, one who is mukallaf (has reached the age in which he/she is responsible for his/her doings and must observe Islamic laws) must choose one of three things:
1) Go after the abovementioned prerequisites and obtain all of them and as a result, reach the high level of ijtihad (Islamic jurisprudence).
2) Go after and learn all of the different scholars’ viewpoints on each issue that comes up and act in a way that all viewpoints are observed regarding each issue in order to be sure that he has fulfilled his duty no matter what it is. Acting in such a manner is called ihtiyat (being cautious).
3) Go to one who has spent his life in thoroughly obtaining the required fields needed for becoming an expert in Islamic laws and rulings.
Without a doubt, if one goes after the first option and achieves it, he will be able to do without the other two, but till reaching ijtihad, he has no other choice but to choose one of the other two.
The second option calls for sufficient awareness of the existing viewpoints on various issues and information on the different methods of ihtiyat. In many cases, ihtiyat causes disorder in one’s everyday life and can get very hard and back breaking.
Inevitably, taqlid is the only option that suits the laity best, in addition to the fact that it might be the only way for the laity. These three methods don’t exclusively belong to fiqh, they are the case in all fields. For example, if an engineer gets sick, he has three ways for getting better; he has to either become a doctor himself, or see what doctors say about his symptoms and act in a way that he won’t regret in the future, or go to an expert (doctor).
The first way won’t get him better soon, and the second is extremely exhausting and won’t let him continue his own major (engineering). Therefore, he has no other choice other than to get help from an expert and follow what he says.
By going to an expert, not only will prevent future regret and scolding by those around him, but he will also most probably get better. Same goes with the mukallaf; if he goes to the experts in fiqh, not only will he free himself of the regret and chastisement of the hereafter, but he will also benefit in this world from the good results of applying Islamic practical laws in his life.
So taqlid means for an unexperienced person in a certain field to go to an expert in that field. The most important reason behind why the laity has to go to religious experts or marjas regarding religious matters is the same simple and understandable reason why all people go to the experts in all fields.
Of course, there is also Quranic and traditional evidence on taqlid being wajib. For instance, the Quran says: “Ask those who know if you don’t know (something).”
Our traditions say: “Concerning the events and issues that come up (that you don’t know the ruling of), refer to the narrators of our hadiths, for they are my (Imam Zaman (a.s.)) hujjahs (hujjah is a person who represents another, what the hujjah says is considered what the person who has appointed him has said, therefore disobeying the hujjah is considered disobedience of his appointer.) upon you and I am Allah’s hujjah upon you.,
Therefore, according to Islam, Ijtihad is a necessity and the only way through which Islam, the real religion, can survive over the history and ages. Shiite jurisprudents and religious authorities differ on the details of secondary issues, not on the fundamentals of faith, tenets of religion, generalities, universal principles and also the conspicuous secondary issues such as prayers, fasting, hajj etc. They are unanimous about many of the sub-issues and laws either.
The conclusion is that Ijtihad is a part of Islam and it must be said that Islam has, somehow, accepted the differences. Of course, the reason as mentioned above is clear. In a nutshell, due to the significance of Ijtihad which rests in the continuity and permanence of the religion, Islam has overlooked some minor discrepancies which may sometimes arise among jurisprudents.
 See: Hadavi Tehrani, The Theological Foundations of Ijtihad, pp. 19 – 20.
 Extracted from question 524 (site: 571) (The Principles of Issuance of Fatwa).
 Sahifeye Noor, v.21, pg. 98.
 Bihar-ul-Anwar, v.1, pg. 214.
 See: Seyyed Mojtaba Hosseini, Resaleye Daneshjooyi, 5th edition, pg. 45-46.
 Wasa’il-ul-Shia, v.27, pg.140.