There is a narration which is distributed to Imam Reza (a.s) regarding the philosophy of Ghusl such as Jinabat, Jomat (Friday) Ghusl and … in which the narrator asked Him: When one had a legal (halal) intercourse and this halal way didn’t make him impure can you please tell us the reason of doing Jinabat Ghusl after? His Excellency replied: "The reason which is behind the Jinabat Ghusl is to clean and purify our soul from its pollution and purify our body from the impurity. This means, if we want to remove the Jinabat from our body we have to purify all parts of our body. What the reason we needn’t do Ghusl after urine and feces is that the both repeat more than Jinabat. Therefore, the holy Lawgiver orders us to make wudhu only, because it would be difficult to do Ghusl. But, since semen comes out with pleasure and excitement it is obligatory on us to do Ghusl."
There has been mentioned some reasons of doing Fitr (the festival at the end of the fasting month) and Qurban eve, Friday Ghusl and etc. that are as under:
First reason: To worship and bow down toward Allah the Almighty to forgive our sins.
Second: since these days are the famous eves in which servants have gathered together and mentioned the name of Allah, Ghusl became the religious rule that people revere these days and considered them preferable to other days.
Third: when people do Ghusl on Friday they would be spiritually pure till next Friday[1].
Since the effect of semen becomes manifest through each parts of our body in which all body's cells will be directed in the special weakest state when it comes out and this way is a sign of its effect on our body, it isn’t looks like the urine and the other excrements which aren’t related to all parts our body but the specialized area of it.
According to scientist's researchers, there are two vegetative nervous system in the human body which control body's activities. The sympathetic nerves and parasympathetic system. The both are exist throughout all parts of our body and around the inner or exterior systems. Activating different systems of our bodies is the responsibility of the sympathetic system but reducing them is specialized for the parasympathetic system.
Sometimes, many events like orgasm (the moment of greatest pleasure and excitement during sexual activity that usually coincide with ejaculation) happened in the human body that disturb this balance. In this parallel condition, the parasympathetic system outruns the sympathetic system and then the balance negatively changed.
It is proved that if water comes into contact with body it can activate the sympathetic nerves and then restore equilibrium. Since, the effect of orgasm can be noticeably seen through all parts of our body and the balance of the both nerves disturb throughout our body it is suggested to wash all parts of body with water after each intercourse or when the semen comes out. As a result, the complete balance has been set up between the both nerves systems.
However, Ghuls has many benefits and not restricted to this one. Because, Ghusl in addition to being a kind of worshipping Allah the Almighty it has many benefits like its moral effect which no one can dispute it. Therefore, if we washed our body not with the intention of proximity to Allah and obey His orders our Ghusl would be considered as incorrect.
Actually, the soul and body are touched while semen is coming out or having intercourse. The soul goes toward the material passion and the body toward weakness and stagnation. But the Jinabat Ghusl, which washes the body and if it is done with the intention of proximity to Allah washes the soul, it produces two effects on the both soul and body simultaneously anyhow that leads the soul toward Allah and spirituality and the body toward purification, activity and jollity.
For further information please refer to the following indexes:
Index: The philosophy and wisdom in regards to the jurisprudential rules, answer 8593 (Website code: 9135).

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