Al-'Illah ṣaḥīḥah ash-shar'iyyah hiya al-'Illah ash-shar'iyyah allatī ṭukirat fī an-nuṣūṣ min al-Qur'ān wa as-Sunnah, li-annahumā waḥdahumā an-nuṣūṣ ash-shar'iyyah. Fa-al-'Illah allatī yurtakazu 'alayhā al-ḥukm ash-shar'ī al-mu'allal hiya 'Illah shar'iyyah wa laysat 'Illah 'aqliyyah. Bi-ma'nā âkhar, yajibu an-tudhkara al-'Illah fī an-naṣṣ ṣarāḥatan aw ḍimnan aw bi-al-istinbāṭ aw ‌ʿan ṭarīq al-qiyās. Wa hādhihi al-'Illah muttaṣilah bi-al-ḥukm al-mu'allal fī al-wujūd wa al-'adam. Wa bi-hādhā tadūru al-aḥkām ma'a 'ilalihā. Fa-najidu anna shay'an yuḥarram fī ḥāl li-wujūd 'Illah shar'iyyah, fa-idhā zālat hādhihi al-'Illah yaṣīru dhālika al-shay' ʿayna mubāḥan. Fa-al-ḥukm ash-shar'ī yadūru ma'a al-'Illah wujūdan wa 'adaman; fa-idhā wujidat al-'Illah wujida al-ḥukm, wa idhā lam takun al-'Illah lam yakun al-ḥukm kadhālika.

The valid „Illah is the Shar‟ai „Illah that is mentioned in the text from Qur'an and Sunnah, for only these two are the Shar‟ai texts. Therefore, the „Illah upon which the reasoned Hukm Shar‟ai is built is a Shar‟ai „Illah and not a rational „Illah. In other words, „Illah must be mentioned in the text either explicitly or implicitly or by deduction or through Qiyas. This „Illah is connected with the reasoned Hukm in presence and absence. Thus the rules revolve with their „Illal. So we find a thing is prohibited in a situation due to the existence of a Shar‟ai „Illah, and if this „Illah disappears that very thing becomes permissible. So the Hukm Shar‟ai revolves with the „Illah in existence and in absence, thus when it exists the Hukm exists, and if the „Illah does not exist the rule doesn't exist either.