Faith in the religion of Islam is based on rational thinking. Quranic teachings always encourage people to achieve faith through reasoning and do not consider mere devotional cognition as adequate. Hence one should accept the Islamic axioms (Monotheism (Tawhid), Prophethood (Nubuwwah), and Afterlife (Ma’ad)) logically.

The above-mentioned Islamic axioms are constant, immutable, and limited, whereas new events and issues of each time are changeable and infinite. Consequently, there needs to be some Islamic scholars or experts who know the Islamic teachings in general and are aware of the contingent issues of the time and their solutions in particular, which makes them responsible for inference of new laws from basic principles of Islam (Ijtihad) in accordance with the needs of changing times and the requirements of new phenomena of human civilization [1].

On the other hand, the Integration of different people to Islam with their particular way of thinking, living with leaders of various religions, the religious discussions between them and the Muslims, and the appearance of Islamic philosophy would always arise doubts and uncertainties. So it necessitated research on the principles of Islam and justifying them especially after the time of the last Prophet (PBUH) and the Imams.

Muslim scholars have always proved that the Islamic teachings are dynamic, compatible with the passage of time, and capable of fulfilling the requirements of each age, generation, and civilization; this would, in consequence, develop the Islamic society and lead it through the path of evolution and perfection in many parts of the world, especially in the first centuries.

Ijtihad

The literal meaning of Ijtihad is to do one's utmost while striving and making effort to reach a goal which in this case is to endeavor to deduce the divine laws of Islam from the reliable sources and proofs, i.e. the Holy Quran, the historical tradition (Sunnah) [i], consensus (Ijma`) [ii], and reason (`Aql).

The term Mujtahid (the religious expert), derived from Ijtihad, refers to a person who endeavors in the way of Allah to derive laws and decrees regarding the religious fundamentals through all kinds of hardships and difficulties.

Ijtihad, which is of great importance in the religion of Islam, guarantees its persistence. Muslims have always been urged to study Islamic science and everything else which is necessary for the development and well-being of their society.

However, it is not compulsory (Wajib) for every single Muslim to become a religious expert (Mujtahid) due to its difficulty and some people’s inability to comprehend and derive Islamic laws all by themselves. It means that the obligation is on the community as a whole and so when a group of people devote themselves to the science of religion to guide the Muslims, then the obligation is lifted from the rest of the society [2].

Quran says: “why should not there go forth a group from each of their sections to become learned in religion, and to warn their people when they return to them, so that they may beware?” (9:122)

It is noteworthy that even though it is a sufficiency duty, every single person in the Islamic community can learn the science of religion and do Ijtihad individually. Therefore this science is not associated with a particular class of the society; rather, it only depends on acquiring the necessary knowledge and intellectual skills. So if a Muslim is not capable of attaining such level of knowledge that would enable him/her to deduce religious laws for himself/herself, it is compulsory for them to refer to an expert who has specialized in this field, i.e., Mujtahid.

Mujtahids

A fully qualified religious expert (Mujtahid), who is supposed to study and deduce the practical laws of Islam according to the time requirements, needs to have specific features, the most significant of which are:

Being able to fully understand the Holy Quran and the other religious sources to discover practical laws from their origins.
Being equitable and trustworthy
Being capable of refraining from sins
Being able to keep away from earthly desires

It is also important to bear in mind that the religious experts (Mujtahids) do not ever issue a decree (Fatwa) unless they have found adequate and reliable proofs and evidence in the Quran, historical tradition (Sunnah), reason, and consensus; which is when they inform the people of God’s commandments.

Imitation (Taqlid) in Islam

Taqlid in Islam literally means "to follow or imitate someone" in the realm of religious do’s and don’ts or the religious laws one must obey. In Islamic terminology, it means to comply with the edicts of a religious expert (Mujtahid) regarding practical affairs of religion.  Broadly speaking, imitation is classified into four different categories among people:

An unlearned following another unlearned
A learned following an unlearned
A learned following another learned
An unlearned following a learned

Quran, however, mentions two of the above; “an unlearned following another unlearned,” which is strictly prohibited:

“For when they are told, "Come unto that which God has bestowed from on high, and unto the Apostle" - they answer, "Enough for us is that which we found our forefathers believing in and doing." Why, even though their forefathers knew nothing, and were devoid of all guidance?” (5:104)

And that of “an unlearned following a learned” (the focus of this article):

“Ask the People of the Book if you do not know” (21:7)

In Islamic thinking, the latter is the only acceptable kind of Taqlid in Islam that appeals to man's rationale. According to common sense, we follow the guidance of a religious expert (Mujtahid) who knows the laws of religion, just as we voluntarily conform to the advice of a doctor when we need medical attention, or in the same way, we consult lawyers and comply with their recommendations. It is inherent in man's nature to resort to experts in fields wherein he lacks expertise.

Practical matters of faith are no different. We, therefore, comply with an expert in the field of practical religious affairs too [3]. In this kind of Taqlid in Islam, which is permitted in Islam, two important elements are involved; firstly, the imitator (Muqallid) must completely trust and have confidence in the religious expert (Mujtahid). Secondly, imitation (Taqlid) must fulfill the imitator’s (Muqallid) demands and lead him/her to perfection. Clearly, this does not make sense in the other forms of imitation (Taqlid) but the last one.

In short, the religious concepts and teachings of Islam fit into two main parts; the axioms and the practical commandments (practical principles). As for the Islamic axioms, i.e., Monotheism (Tawhid), Prophethood (Nubuwwah), and Afterlife (Ma’ad), no one is allowed to imitate; instead, each person is supposed to investigate and accept them individually since they are regarded as the main entrance to the religion of Islam.

But about Practical principles, which are obligatory practical commandments, Muslims are encouraged to investigate and find them out as individuals if they are able to do so; obviously, they are not allowed to imitate anyone. If they are not capable, though, they have to follow religious experts (Mujtahid) who have become specialized in Islamic science fully and deeply.

It is learned in this article that the cases in which Taqlid in Islam or imitation is allowed, are very limited in Islam. In fact, it is possible for every single Muslim to step on the path of investigation to attain knowledge and awareness about the truth and commandments of Islam themselves.

Notes:

[i]. the primary source of law taken from the sayings, actions, and approvals of the Prophet Muhammad (PBUH)

[ii]. acceptance of a matter by a specified group of Muslim scholars

References:

1. Ijtihad: Its Meaning, Sources, Beginnings and the Practice of Ra'

2. Taqleed

Full body ablution (Ghusl) refers to an Islamic ritual in which an adult (Mukallaf) Muslim is recommended to wash his/her full body with specific rulings. 

In this article, we will explain different types of full-body ablution (Ghusl) and their rulings.

Different types of full-body ablution (Ghusl)

Muslim jurists have driven the rules of full-body ablution (Ghusl) based on different verses of the Quran and different narrations and traditions. The main verse of the Quran that is referred to in deriving the rules of full-body ablution (Ghusl) is the following: 

“O you who have faith! Do not approach prayer when you are intoxicated, [not] until you know what you are saying, nor [enter mosques] in the state of ritual impurity until you have washed yourselves, except while passing through. But if you are sick or on a journey, or any of you has come from the toilet, or you have touched women, and you cannot find water, then make your full-body ablution on clean ground and wipe a part of your faces and your hands. Indeed Allah is all-excusing, all-forgiving.” (4:43)

Based on the above verse, Muslim jurists have provided the following rules for full-body ablution (Ghusl):

The obligation of full-body ablution (Ghusl)

Different types of full-body ablution (Ghusl) can be categorized as follows: 

1-    The obligatory full body ablution (Ghusl) common between men and women:

a)    The full-body ablution (Ghusl) for ritual impurity (Janabah)
b)    The full-body ablution (Ghusl) for touching a corpse (Mass al-Mayyit)
c)     The full-body ablution (Ghusl) given to a corpse (Mayyit) 
d)     The full-body ablution (Ghusl) that becomes obligatory on the account of a vow (Nazr), oath (Qassam), etc.

2-    The obligatory full-body ablution (Ghusl) for women: 

a)     The full-body ablution (Ghusl) for menstruation (Haydh)
b)      The full-body ablution (Ghusl) for lochia (Nifas)
c)      The full-body ablution (Ghusl) for irregular blood discharge (Istihadhah) [1]

3-    Recommended full body ablution (Ghusl)

There are some specific times that Muslims are recommended to perform full-body ablution (Ghusl), for example, the Friday Full-body ablution (Ghusl-e Jum’ah), or the full-body ablution (Ghusl) that are recommended to perform on specific Islamic occasions such as the nights of decree. (Laylatul Qadr). [2]

How to perform full-body ablution (Ghusl)?

There are two methods to perform full-body ablution (Ghusl) that will be described in this part. The first thing to do before starting the full-body ablution (Ghusl) is to make an intention for it. However, it is not necessary to perform a separate full-body ablution (Ghusl) for different intentions. 
If there are several recommended (Mustahab) or obligatory full-body ablutions (Ghusl) to be performed and one performs one full-body ablution (Ghusl) with the intention of performing all of them, it is sufficient. However, if one of them is full-body ablution (Ghusl) of ritual impurity (Janabah) and the intention is made to perform it, it suffices for all other full-body ablutions (Ghusl), although caution is to make the intention for all of them. [3]

The methods of full-body ablution (Ghusl)

1-    Sequential full-body ablution (Ghusl-e Tartibi)

In sequential full-body ablution (Ghusl), one must – based on obligatory precaution – first, with the intention of full-body ablution (Ghusl), wash the entire head and neck and then the entire body. It is better to first wash the right side of the body, then the left. If one intentionally or due to being negligent in learning the laws of full-body ablution (Ghusl)does not wash the entire head and neck before washing the body, then based on obligatory precaution his full-body ablution (Ghusl)is invalid. Furthermore, based on obligatory precaution, when performing full-body ablution (Ghusl), it is not sufficient to make the intention of full-body ablution (Ghusl) when moving the head, neck, or body while they are already under the flow of water; rather, the part that one wants to perform full-body ablution (Ghusl)on – on the condition that it is already under the flow of water – must be taken out from under the flow of water and then washed with the intention of full-body ablution (Ghusl). [4] 

2-    Immersive full-body ablution (Ghusl-e Irtimasi) can be performed in two ways:

a)    Instantaneous (Daf’i): 

In this type of full-body ablution (Ghusl) water must cover the entire body in one go. However, it is not necessary for the entire body to be out of the water before starting the full-body ablution (Ghusl): rather it will suffice if part of the body is out of the water and the person goes under the water completely with the intention of performing full-body ablution (Ghusl). An example of instantaneous immersive full-body ablution (Ghusl)is when a person, with the int¬ention of performing full-body ablution (Ghusl), dives/jumps into a swimming pool and in doing so completely immerses himself in the water; or, the person may already be partially immersed in the water and he then completely immerses himself with the intention of full-body ablution (Ghusl). [5]

b)    Gradual (Tadriji)

In gradual immersive full-body ablution (Ghusl), one must gradually – but in a way that can be commonly considered to be one unified action – immerse his body in water with the intention of full-body ablution (Ghusl). In this type of full-body ablution (Ghusl), it is necessary for each part of the body to be out of the water before it is washed. An example of gradual immersive full-body ablution (Ghusl) is when a person, with the intention of performing full-body ablution (Ghusl), immerses part of his body in a bathtub of water and then takes that part out of the water; then, he immerses another part of his body and takes it out, and so on until all the parts of his body have been immersed. [6]

Water requirements for full-body ablution (Ghusl)

The water that is permissible for full-body ablution (Ghusl) can be Rainwater, Well water, water from spring, sea, or river water, water of melting snow or hail, the water of a big tank or pond. Ghusl is not allowed with unclean or impure water or water extracted from fruit and trees. [7] 

References: 

1. Ghusl
2. ablution
3. ghusl
4. Full body ablution
5. obligatory baths
6. bathing
7. Imam Khomeini, Risalah Tozih al-Masa’el, p. 39-40