The Reason Why Ghusl (Ritual Bath) is Obligatory for Janabah (Ritual Impurity) but Not Urine and Feces
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 18
Man Lā Yaḥḍuruh al-Faqīh
Book 1, Chapter 18
The Reason Why Ghusl (Ritual Bath) is Obligatory for Janabah (Ritual Impurity) but Not Urine and Feces
وَ كَتَبَ اَلرِّضَا عَلَيْهِ اَلسَّلاَمُ إِلَى مُحَمَّدِ بْنِ سِنَانٍ فِيمَا كَتَبَ مِنْ جَوَابِ مَسَائِلِهِ: «عِلَّةُ غُسْلِ اَلْجَنَابَةِ اَلنَّظَافَةُ لِتَطْهِيرِ اَلْإِنْسَانِ مِمَّا أَصَابَ مِنْ أَذَاهُ وَ تَطْهِيرِ سَائِرِ جَسَدِهِ لِأَنَّ اَلْجَنَابَةَ خَارِجَةٌ مِنْ كُلِّ جَسَدِهِ فَلِذَلِكَ وَجَبَ عَلَيْهِ تَطْهِيرُ جَسَدِهِ كُلِّهِ وَ عِلَّةُ اَلتَّخْفِيفِ فِي اَلْبَوْلِ وَ اَلْغَائِطِ أَنَّهُ أَكْثَرُ وَ أَدْوَمُ مِنَ اَلْجَنَابَةِ فَرَضِيَ فِيهِ بِالْوُضُوءِ لِكَثْرَتِهِ وَ مَشَقَّتِهِ وَ مَجِيئِهِ بِغَيْرِ إِرَادَةٍ مِنْهُ وَ لاَ شَهْوَةٍ وَ اَلْجَنَابَةُ لاَ تَكُونُ إِلاَّ بِالاِسْتِلْذَاذِ مِنْهُمْ وَ اَلْإِكْرَاهِ لِأَنْفُسِهِمْ.
Imam Ali ibn Musa Ar-Ridha عليه السلام wrote to Muhammad ibn Sinan in response to his questions: 'The reason for the ghusl (ritual bath) of janabah (ritual impurity from sexual discharge) is for cleanliness, to purify a person from what has affected them and to cleanse their entire body, because janabah involves the whole body. Therefore, it is necessary to purify the entire body. The reason for the lightening (of obligation) in the case of urine and feces is because they occur more frequently and continuously compared to janabah, so it is sufficient to perform wudu (ablution) due to their frequency, difficulty, and the fact that they occur without intentional desire or pleasure. Janabah occurs only through enjoyment or intentional acts involving themselves.'
- Ṣaḥīḥ al-Faqīh