I. 'Ilmu 'I-Usul, or the Exegesis of the Qur’an, is a very important science, and is used by the Muslim divine to explain away many apparent or real contradictions. The most authoritative works on the' llmu- Usul of the Qur'an, are Manaru 'I-Usul and its commentary, the Nuru 'I-Anwar, and as-Suyuti's Itqan (ed. by Sprenger). The various laws of interpretation laid down in these books are very complicated, requiring the most careful study. We have only space for a mere outline of the system. 

The words (alfaz) of the Qur’an are of four classes: Khass, 'Amm, Mushtarak, and Mu'awwal. 

(1) Khass Words used in a special sense. This specialty of sense is of three kinds: Khususu'l-jins, Specialty of genus, e.g. mankind; Khususu 'n-nau', Specialty of species, e.g. a man; khususu'l -'ain, Specialty of an individual, e.g. Muhammad. 

(2) 'Amm Collective or common, which embrace many individuals or things, e.g. people. 

(3) Mushtarak Complex words which have several signification e.g. ain, a word which signifies an Eye, a Fountain, the Knee, or the Sun. 

(4.) Mu'awwal words which have several significations, all of which are possible, and so a special explanation is required. For example, Surah cviii. 2, reads thus in Sale's translation. "Wherefore pray unto the Lord and slag (the victims)." The word translated "slay" is in Arabic inhar, from the root nahr, which has several meanings. The followers of the great Legist, Abu Hanifah, render it "sacrifice," and add the words (the "victims "). The followers of Ibn Ash- Shafi'i say it means" placing the hands on the breast in prayer." 

II. The Sentences ('Ibarah) of the Qur'an are either Zahir or Khafi, i.e. either Obvious or Hidden.  Obvious sentences are of four classes: Zahir, Nass, Mufassar, Muhkam.

(1.) Zahir Those sentences, the meaning of which is obvious or clear, without any assistance from the context (qarinah). 

(2.) Nass, a word commonly used for a text of the Qur'an, but in its technical meaning here expressing what is meant by a sentence, the meaning of which is made clear by some word which occurs in it. The following sentence illustrates both Zahir and Nass: "Take in marriage of such other women as please you, two, three, four wives." This sentence is Zahir, because marriage is here declared lawful; it is Nass, because the words "one, two, three, four," which occur in the sentence, show the unlawfulness of having more than four wives. 

(3.) Mufassar, or explained. A sentence which needs some word in it to explain it and make it clear. Thus:" And the angels prostrated themselves, all of them with one accord, save Iblis (Satan)." Here the words "save Iblis " show that he did not prostrate himself. This kind of sentence may be abrogated. 

(4.) Mukham, or perspicuous. A sentence as to the meaning of which there can be no doubt, and which cannot be controverted, thus: "God knoweth all things." This kind of sentence cannot be abrogated. To act on such sentences without departing from the literal sense is the highest degree of obedience to God's command. 

The difference between these sentences is seen when there is a real or apparent contradiction between them. If such should occur, the first must give place to the second, and so on. Thus Muhkam cannot be abrogated or changed by any of the preceding, or Mufassar by Nass, etc.