In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
Allaah (The Most High) said:
الٓرۚ تِلۡكَ ءَايَـٰتُ ٱلۡكِتَـٰبِ ٱلۡمُبِينِ
إِنَّآ أَنزَلۡنَـٰهُ قُرۡءَٲنًا عَرَبِيًّ۬ا لَّعَلَّكُمۡ تَعۡقِلُونَ
Alif-Laam-Raa. These are the Verses of the Clear Book. Verily, We have sent it down as an Arabic Qur’an in order that you may understand. [12:1-2]
Allaah (The Most high) informed (us) that the verses of the Qur’aan are clear; meaning: Its wordings and meanings are manifest and clear. And from its manifest and clear (nature) is that it has been sent down in the Arabic Language—the noblest and most (precise and eloquent of all languages in giving clarification, explanation etc). And all this clarity and explanation is so that لَّعَلَّكُمۡ تَعۡقِلُون — ‘’You may understand’’; meaning so that you may understand its prescribed boundaries (in punishments, retribution, acts of worship, dealings etc); its fundamental principles; its subsidiary issues; its commandments and prohibitions. So when you understand it (after studying, contemplating etc) and your hearts become characterized with its understanding, then that will produce the fruits of actions of the limbs and submission to it. And لَّعَلَّكُمۡ تَعۡقِلُون – ‘’In order that you may understand’’; meaning: Your intellects are enhanced by way of affirming the noble and lofty meaning (of affairs) in your minds; so you move from one state to another loftier and more perfect state. 
Allaah [The Most High] said:
تَنزِيلٌ۬ مِّنَ ٱلرَّحۡمَـٰنِ ٱلرَّحِيمِ
كِتَـٰبٌ۬ فُصِّلَتۡ ءَايَـٰتُهُ ۥ قُرۡءَانًا عَرَبِيًّ۬ا لِّقَوۡمٍ۬ يَعۡلَمُونَ
Haa-Meem. A revelation from Allah, the Most Beneficent, the Most Merciful. A Book whereof the Verses are explained in detail; A Qur’an in Arabic for people who know.” [41:1-3]
Allaah (The Most High) informed His slaves that indeed this lofty Book and beautiful Qur’aan is a [تَنزِيلٌ۬ ] revelation [مِّنَ ٱلرَّحۡمَـٰنِ ٱلرَّحِيم—from the Most Beneficent, the Most Merciful] Whose Mercy encompasses everything. And from His greatest and abundant mercy is the sending down of this Book through which knowledge and guidance, light and cure, mercy and abundant good are received. There is no greater favour of His upon the slaves than it, and it is the path towards happiness in both worlds (i.e. this life and the next).
Then He (Allaah) praised the Book saying that it gives a complete explanation: [فُصِّلَتۡ ءَايَـٰتُهُ]; that is: It explained every matter based on its differences and exact (perfect) determined boundaries, and this necessitates a complete explanation, distinction between everything and a distinction between the different affairs of reality. [قُرۡءَانًا عَرَبِيًّ۬ا -A Qur’an in Arabic]: That is: In the pure (Arabic) language-the most perfect of languages. Its verses have been explained and revealed in the Arabic language [لِّقَوۡمٍ۬ يَعۡلَمُونَ- for people who know]; so that its meanings are made clear to them just as its wordings are clear; (and so that) guidance is made clearly distinct from misguidance and being misled is made distinct from being led aright.
As for the (obstinate) ignorant ones- those who (in the presence) of guidance are not increased in aught except misguidance; those who (in the presence) of clarification are not increased in aught except blindness, then speech is not uttered due to them; [سَوَآءٌ عَلَيۡهِمۡ ءَأَنذَرۡتَهُمۡ أَمۡ لَمۡ تُنذِرۡهُمۡ لَا يُؤۡمِنُونَ — It is the same to them whether you (O Muhammad) warn them or do not warn them, they will not believe]. 
Allaah (The Most High) said:
الٓرۚ كِتَـٰبٌ أُحۡكِمَتۡ ءَايَـٰتُهُ ۥ ثُمَّ فُصِّلَتۡ مِن لَّدُنۡ حَكِيمٍ خَبِيرٍ
Alif-Laam-Raa. (This is) a Book, the Verses whereof are perfected, and then explained in detail from One (Allah), Who is All-Wise and Well-Acquainted (with all things).
Allaah (The Most High) said: This is a great Book and a noble revelation. [أُحۡكِمَتۡ ءَايَـٰتُهُ– the Verses whereof are perfected], meaning perfect and excellent. Its information is (absolute) truth and its commandments and prohibitions are (absolutely) just. Its wordings are (absolutely) eloquent and its meanings are (absolutely) magnificent. [ثُمَّ فُصِّلَتۡ– And then explained in detail]; meaning: Distinguished and explained with the loftiest of explanations. [مِن لَّدُنۡ حَكِيمٍ خَبِير— From One (Allah), Who is All-Wise and Well-Acquainted (with all things)]. He (Allaah) establishes all things (or all affairs) in their rightful places and positions them in their rightful positions. He neither commands nor prohibits except in accordance with His (perfect) Wisdom. [خَبِير]- ‘Well-Acquainted’ with all apparent and hidden things.
And when it is the case that its (i.e. the verses of the Qur’aan) perfection and detailed explanation is from Allaah, The All-Wise and Well Acquainted (with all hidden and apparent things), then no question should be asked about its greatness and loftiness, and about it being gathered upon perfect wisdom and vast mercy. 
 [An Excerpt from ‘Tayseer Al-Kareem Ar-Rahmaan Fee Tafseer kalaam Al-Mannaan’ of Imaam Sadi (rahimahullaah). Slightly paraphrased]
 [An Excerpt from Tayseer Al-Kareem ar-Rahmaan Fee Tafseer Kalaam al-Mannaan of Imaam Sadi (rahimahullaah). Slightly paraphrased]
 [An Excerpt from Tayseer Al-Kareem ar-Rahmaan Fee Tafseer kalaam Al-Mannaan’ of Imaam Sadi. Slightly paraphrased]