Why does body become Najis after soul leave the body and if it is Najis, then how does Ghusl e Mayyit purifies the body? 

Short Answer:

Hadath Asghar and Akbar: Something that violates purity (Wudu, ghusl, and Taymmum) is called Hadath. It is of two types: Minor Hadath; such as Boul (urine), Gha’et (faeces), and Major Hadath; like semen exit, sexual intercourse, menstruation, etc.

The philosophy of Ghusl in time of Hadath Akbar (when alive) is to bring human (body and soul) back to balance after it has experienced some sort of spiritual and psychological imbalance in times of Hadath. Death is also a major Hadath.

Detailed Answer 1 (Scientific)

 According to scientists’ research, there are two types of plant nerves in the human body that control all the body’s activities, “sympathetic nerves” and “parasympathetic nerves”.

 The function of the sympathetic nerves is to make different body systems work, and the function of the “parasympathetic” nerves is to slow down their activity. Sometimes currents occur in the body that disturb this balance, among these currents is at the peak of intimate relationship or Janaabat. An imbalance similar to that (although different) also occurs during death and the departure of the soul from the body. At this time, the parasympathetic nervous system (inhibiting nerves) overcomes the sympathetic nerves (stimulating nerves) and the balance is disturbed in a negative way.

It has also been proven that one of the things that can activate the sympathetic nerves and provide the lost balance is the contact of water with the body. Since the effect of ejaculation and menstruation on all body parts can be seen noticeably and the balance of these two groups of nerves is disrupted throughout the body, it has been ordered that after these two events (for example) the whole body should be washed with water and In the light of the life-giving effect of water, a complete balance between those two groups of nerves throughout the body should be established.

Such a state is also formed for the soul and body at the time of departure from body, and the soul experiences stress and imbalance when leaving the body (whether in sleep or death). For this reason, they bathe the body, firstly to clean it of external dirt, and secondly, to remove the spiritual imbalance of the dead. However, because the imbalance of a person during sleep is less than the state of death (the complete departure of the soul from the body), Wudu (small Ghusl) has been ordered for that.

You may ask, what effect does the water sprinkled on the body have on the soul that is separated from the body?

In response, we say that at the time of death, the soul still has a relative interest in the body and is connected with it and suffers from the pollution of the body and enjoys the connection of a pure thing (water) with it and feels balance and peace.

Detailed Answer 2 (Ontological)

The world of Mulk and Malakut: The former is the physical world; the latter is the spiritual world.

Najis is spiritually impure (Khubth Batin). It is spiritually low so is incompatible to spiritually high things. It is something that is away from the world of Malakut (Akhirah) and is very close to the world of Mulk (Dunya). It does not mean being useless. For example, if blood, dogs or pigs are Najis, it does not mean that they are useless, but they are away from the world of Malakut. There is also physically unclean thing, which is the pollution, and dirt that we all know. Spiritually low entetity is everything that is far from God and from the realm of Malakut. Among the creatures of the universe, there are 10 of them which are distant from the world of spirit and God, such as urine, blood, semen, wine, dogs and pigs, corpses, etc. (why these are spiritually unclean is another matter). Well, one of these impurities is human corpse without soul.

Why the human body is spiritually unclean after death is not explained in detail in the verses and hadiths (like many of the reasons for rulings), but in general it should be known that the human body is not impure as long as there is a soul in it, because the soul is from the realm of Malakut. (Except in cases where minor events-Hadath Asghar such as sleep, passing urine and faeces, etc., or major events –Hadath Akbar- such as sexual intercourse occur for him, in which case he must perform Ghusl, but as soon as it is separated from the soul, it is nothing but blood and flesh. It is clear that blood (for example) is impure from the point of view of jurisprudence, and from the point of view of ontology, it is from the low enteritis of Mulk.

Now, When the human soul leaves the body, it enters into another ecstasy, and during this transition, difficult things happen to it, such as fear and anxiety, even physical and external pollution. Both because it is attached to the body and because it is supposed to meet God, the Ahl al-Bayt, the angels, and the believers, it must be free from these impurities, just as when became dirty (physically& spiritually) in the world, it must be purified to worship God or to do any highly spiritual act.

Finally, at the time of death, both the minor and the major Hadath occur for a human being, and in order for a person to come out of both the external and internal pollution (which was mentioned) and meet God clean, he must be bathed, that is, with water in a special way to purify which is called Ghusl. It is water because water is the best cleanser. It is in a special way because that special way has a special feature that causes spiritual purity. That’s why they say that Wudu and Ghusl should be done in the same way as mentioned in the religion, otherwise it might have physical cleansing effect, but not a spiritual one.