Muhammad Asad, The Message of the Qur’ān; Translated and Explained by Muhammad Asad (1980)

are many sūrahs which open with an explicit reference to divine revelation and are nevertheless not preceded by any letter-symbol. Secondly – and this is the most weighty objection – the above explanation, too, is based on no more than conjecture: and so, in the last resort, we must content ourselves with the finding that a solution of this problem still remains beyond our grasp. This was apparently the view of the four Right-Guided Caliphs, summarized in these words of Abū Bakr: “In every divine writ (kitāb) there is [an element of] mystery – and the mystery of the Qurʾān is [indicated] in the openings of [some of] the sūrahs.”