الٓر تِلْكَ ءَايَٰتُ ٱلْكِتَٰبِ ٱلْمُبِينِ · إِنَّآ أَنزَلْنَٰهُ قُرْءَٰنًا عَرَبِيًّا لَّعَلَّكُمْ تَعْقِلُونَ
"Alif-Lām-Rā. These are the verses of the clear Book. We have sent it down as an Arabic Qurʾān, that you may reason."
The note on the disconnected letters has been entered many times in the index; I do not repeat it here.
The same opening recurs in several sūras of the Qurʾān and was tabulated at al-Shuʿarāʾ 26/2: tilka āyātu'l-kitābi'l-ḥakīm (Yūnus 10/1), tilka āyātu'l-kitābi'l-mubīn (here and al-Shuʿarāʾ 26/2), tilka āyātu'l-kitābi'l-ḥakīm (Luqmān 31/2). I rely on that treatment.
ٱلْمُبِين — root ب-ي-ن, an active participle of Form IV. It must be recorded that the pattern can be read in Arabic as both transitive and intransitive: mubīn means both "that which is clear" and "that which makes clear". The root has been treated in many places in the index.
I record this as an observation, and it has a particular place in this sūra: the story to be told is built from beginning to end on things hidden and then brought to light — the dream that is kept back (5), the brother who is concealed (58), the word that is hidden (77), and at last al-ʾāna ḥaṣḥaṣa'l-ḥaqq (51). The sūra opens as a "clear Book" and proceeds through scenes of coming to light.
This sentence is word for word the same at al-Zukhruf 43/3, and it was treated at al-Zukhruf 43/3; there all the places where the Qurʾān describes itself as "Arabic" were listed (Ṭāhā 20/113, al-Zumar 39/28, Fuṣṣilat 41/3, al-Shūrā 42/7, al-Aḥqāf 46/12, al-Raʿd 13/37). The same link was made at al-Shūrā 42/7 as well. I do not repeat it here.
The only thing that belongs here is the context, and I give it as my own reading: that "so that you may reason" should stand at the head of a narrative sūra is worth recording. The story is offered not as a tale to pass the time pleasantly with, but as material to reason upon — and the sūra will repeat this in its final verse: ʿibratun li-ūli'l-albāb (111).