Protection From Any Alteration

The transmission of the Qur'an, from the day of its revelation up to the present day, is flawless. The chapters and verses have been in constant use amongst Muslims and have been passed on perfectly intact from one generation to the other. The Qur'an we know today is the same Qur'an which was revealed to the Prophet some fourteen centuries ago.
The Qur'an does not stand in need of historical proof for its identity or authenticity, (although history too confirms its validity). Since a book which claims to be the actual unalterable word of God and attests to this in its own text, does not need to resort to others to prove its authenticity. The clearest proof that the Qur'an we have with us today is the same that was revealed to the Prophet and that no alteration has taken place in its text is that very superiority which the Qur'an claimed for itself at the time of its revelation and which still exists.
The Qur'an says that it is a book of light and guidance, a book which shows man the truth and reality of existence; it says that it explains all things, that is, everything necessary for man to live in accordance with his own natural character; it says that it is the word of God and challenges man and jinn to produce similar words; -it invites them to find someone like the Prophet, who could neither read nor write and grew up in an age of ignorance as an orphan without instruction; the Qur'an challenges them to find any inconsistency in its method, Sciences, or laws, such as one might find in any ordinary book. They obviously cannot for the superiority of the Qur'an remains after its revelation.
Likewise, the guidance for man contained in the Qur'an is still valid; it still expounds a complete world view which is in accord with the purest of intellectual proofs and is the source of man's well being in this world and in the next. By the benevolence and care shown by the Creator for His creation in the Book, it still invites man to belief. The Qur'an cares for the needs of man by giving him a vision of reality based on Divine Unity. All knowledge and belief spring from this view of reality. At no point does the Qur'an fail to explain in the most comprehensive fashion the reality of this oneness. It devotes much attention to explaining the behavior and transactions expected of the individual in society and shows how correct action is that which accords with the natural character and capability (fitrah) of man.
The Qur'an leaves the detailed description of man's behaviour to the Prophet whose daily life was an example of how man was to apply what was contained in the Qur'an. Together the Book of God and the example (or Sunnah) of the Prophet delineated an astoundingly comprehensive life-pattern for man, namely, the way of living in tune with the reality which is Islam. The Qur'an deals precisely with all aspects of individual and social life and, despite having been revealed in another age, does not contain the slightest inconsistency or in- compatibility even today. It describes a din, a comprehensive way of life, whose programme of living is beyond the imagination of the world's most capable lawyers and sociologists.
The miracle of the Qur'an has in it clarity and eloquence, rooted, as it is, in the language of a nation famed for the purity and power of its language. The Qur'an is a miraculous sun whose light shines far brighter than the finest poetry of the time, indeed of any age. During the Islamic conquests of the first century after Hijra, the resulting admixing of non-Arabic words with the Arabic lessened the purity of Arabic language used in the Qur'an causing it to disappear from the every-day speech of the people. The Qur'an does not merely challenge man by the use of its language but also by the depth of its meaning. Those familiar with the Arabic language (both prose and verse writings) are reduced to silence and astonishment when they attempt to describe it. The Qur'an is neither poetry nor prose but rather seems to draw qualities from both; it is more attractive and dazzling than poetry and clearer and more flowing than prose.
A single verse or phrase from the Qur'an is more illuminating, more penetrating, and more profound than the complete speech of most eloquent speakers. The profundity of meaning in the Qur'an remains as miraculous as ever; its complex structure of beliefs, morals and laws stands as proof that the Qur'an is the word of God. Man, and in particular someone who was born and raised in circumstances similar to those of the Prophet, could never have created such a system; the Qur'an is a harmonious whole despite having been revealedduring twenty-three years in greatly varying circumstances. God Himself confirms that the Qur'an has been preserved from change; in chapter XV:9 He says,
"Indeed We, even We, reveal the Reminder and indeed We are truly its guardian," and in chapter (XLI:41-42)
He says, 'for indeed it is an unassailable Book. Falsehood cannot come to it from before or behind it. (It is) a revelation from the Wise, the Praise one." Only a divine Book could remain preserved for fourteen centuries in a world where the enemies of truth and of Islam are numerous.
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(Allamah Tabatabai, The Qur'an in Islam, p. 101-103)