"And We put coverings over their hearts (and minds) lest they should understand the Qur'an, and deafness into their ears: when thou dost commemorate thy Lord and Him alone in the Qur'an, they turn on their backs, fleeing (from the Truth).
 

(Holy Qur'an (Yusufali) 17:46)

"Those who listen to the Word, and follow the best (meaning) in it: those are the ones whom God has guided, and those are the ones endued with understanding."
 

(Holy Qur'an (Yusufali) 39:18)

"He giveth wisdom unto whom He will, and he unto whom wisdom is given, he truly hath received abundant good. But none remember except men of understanding."
 

(Holy Qur'an (Pickthall) 2:269)

"This world is a thing of great and exclusive importance to those who have no breadth of vision, and who cannot see or substantialize what lies behind it and beyond it. The man with true insight of things realizes that not this but the next world is the place to rest and to receive rewards. (The one) who understands realities does not care to work for this world only, but one who cannot comprehend them concentrates his mind and energies on this life only.
 
Verily a real insight in the philosophy of life and death is the source of knowledge which will bestow new lives to minds dead to actualities, will give vision to eyes blind to realities, will confer power of hearing to ears deaf to the inner voice of conscience and reasoning, and will fully satisfy the thirst for greater knowledge.
 
Remember that for one who sincerely and intelligently accepts religion there lies complete emancipation from legal, social, political and intellectual control of heathenism, superstitions and taboos, on it is based full freedom from the sway of the Satan and its allurements and therein lies real peace and true comforts.
 
Your salvation is guaranteed by the book of God. With its help you will recognize the way He wants you to follow; through it you will speak the truth; from it you will hear the voice of reason and truth; its difficult or dogmatic sayings will explain each other and will bear testimony to their rationality and continuity; in explaining His commands and interdictions there are contradictions in its verses and it will not misguide or misinform those who try to understand and to follow it."
 

(Nahjul Balagha: Sermons, letters and saying of Hazrat Ali (Second American edition, 1981), Khutba 136, p. 101)