It is not an exaggeration to claim that Islam emphasizes on reason and knowledge more than any other religions of the world. A cursory study of the Holy Qur'an reveals that Allah has addressed the intellect of men in many different ways:

He grants wisdom to whom He pleases, and whoever is granted wisdom, he indeed is given a great good and none but men of understanding mind (Qur'an 2:269)

Most surely in the creation of the heavens and the earth and the alternation of the night and the day there are signs for men who understand (Qur'an 3:190)

And in many other verses in the Holy Qur'an.

The Holy Qur'an emphasizes that one should pursue only that of which he has a definite and clear knowledge:

And follow not that of which you have not the knowledge; surely the hearing and the sight and the heart, all of these, shall be questioned about that. (Qur'an 17:36)

Such a knowledge is obtained through convincing and clear proof:

You have no authority for this; do you say against Allah what you do not know? (Qur'an 10:68)

These are their vain desires. Say: Bring your proof if you are truthful (Qur'an 2:111)

Guess and conjecture do not lead to such a knowledge:

And most of them do not follow (anything) but conjecture; surely conjecture will not avail aught against the truth; surely Allah is cognizant of what they do (Qur'an 10:36)

From the point of view of the Holy Qur'an conjecture has no value at all. In several verses it has been described as senseless and blind action:

Those who are polytheists will say: If Allah had pleased we would not have associated (aught with Him) nor our fathers, nor would we have forbidden (to ourselves) anything; even so did those before them reject until they tasted Our punishment. Say: Have you any knowledge with you so you should bring it forth to us? You only follow a conjecture and you only tell lies (Qur'an 6:148)

And if you obey most of those in the earth, they will lead you astray from Allah's way; they follow but conjecture and they only lie. (Qur'an 6:116)

And they say: "What is there but our life in this world? We shall die and we live, and nothing but time can destroy us." But of that they have no knowledge: they merely conjecture (Qur'an 45:24)

They are naught but names which you have named, you and your fathers; Allah has not sent for them any authority. They follow naught but conjecture and the low desires which (their) souls incline to (Qur'an 53:23)

The Holy Qur'an has mentioned some basic organs through which reliable knowledge is obtained. They are: Ears for hearing, Eyes for seeing and Heart for understanding:

And Allah has brought you forth from the wombs of your mothers-- you did not know anything-- and He gave you hearing and sight and hearts that you may give thanks (Qur'an 16:78)

Then He made him complete and breathed into him of His spirit, and made for you the ears and the eyes and the hearts; little is it that you give thanks. (Qur'an 32:9)

One of the main sources of our knowledge is hearing, through which we come to know of the experience, investigations, and the ideas of others. We hear of many events from other individuals and other reliable sources.

Another main source of our knowledge is seeing and observation.

The third source is inner perception and comprehension. The knowledge which is obtained through seeing, hearing and inner observation still remains superficial and has little value until it is further analyzed. This raw material must be processed in the region of heart so that it may become reliable, valuable and fit for being accepted and followed.

According to the Holy Qur'an the maturity of man depends on the correct use of these faculties. If these faculties are not used properly, man sinks to the level of animals:

And certainly We have created for hell many of the jinn and the men; they have hearts with which they do not understand, and they have eyes with which they do not see, and they have ears with which they do not hear; they are as cattle, nay, they are in worse errors; these are the heedless ones (Qur'an 7:179)

The Messenger of Allah (SAW) said:

Almighty God has endowed upon mankind nothing better than reason (wisdom). Sleep of a wise man is better than the waking hours of an ignorant. Rest of a wise man is better than the movement (journeys for Hajj or Jihad) of ignorant. God has not send any prophet or messenger without first perfecting his reason; and his reason stands superior to all the reasons of (the people of) his community. What the Prophet has wished is preferable to all the Ijtihad (striving) of the Mujtahideen (those who strive).

No creature can ever discharge his obligations to God unless he comprehends those (obligations). All the worshippers weighted together can not reach the height of excellence of a wise man. The man of reason who are the possessors of understanding minds about whom Allah said: 'But non will grasp the Message except men of understanding'. [2:269, 3:7] (Usul al-Kafi)

Following is a conversation between  Imam Musa al-Kadhim, and one of his faithful disciples, Hisham Ibn al-Hakam as recorded in al-Kafi. Hisham was well-known for his exceptional ability in debate with the polytheists and the members of the heretical groups.

Hisham Ibn al-Hakam narrated that Imam Musa Ibn Jafar addressed him, saying:

O' Hisham! Almighty God has given glad tidings to the people of reason and understanding to this effect:

Glad tidings unto My servants! Those who hear the sayings and follow the best of it; those are who Allah has guided and those who are the possessors of understanding minds. [Qur'an 39:17-18]

O Hisham! It is through reason and understanding that God has completed evidence of proof (in respect of Himself and His religion) for mankind. He has helped His prophets and His messengers by endowing them with the gift of eloquence and guided them to comprehend His lordship through reason and understanding. As He himself has said:

And your Lord is one God. There is no God but He, the All-Merciful, the Compassionate. Surely in the creation of the heavens and the earth, and the alteration of night and day and the ship that runs in sea with profit to men, and the water God sends down from heaven by which He revives the earth after its death and His scattering abroad in it all kinds of crawling creatures, and the turning about of the winds and the clouds suspended between the heaven and the earth - surely there are signs for the people of understanding. [Qur'an 2:163-164)

O' Hisham! Remember that reason is in alliance with knowledge, as God has said:

And those examples we strike for people, but none comprehends them except those who have knowledge. [Qur'an 29:43]

In this part the Imam talks about the meaning of heart in Qur'an. Imam Musa al-Kazim continued:

O' Hisham! Almighty God has said in His book: "Surely in that there is a reminder to him who has a heart [Qur'an 50:37]". Heart means  reason. And God has further said: "Indeed we gave Wisdom to Luqman. [Qur'an 31:12]" Wisdom means (the capacity of) understanding and reasoning.

O' Hisham! Luqman the Sage, counseled his son: "If you bow before the Truth and the Inevitable, you will be the wisest among people, since a wise man treats himself as little before the Truth and the Inevitable. O' my son! This world is like an unfathomable ocean in which many people have drowned. Make, therefore, the fear of God as your boat in this ocean, your faith as the main plank of the boat, the reliance of God as its sails, the reason as its rower, the knowledge as its Captain, and patience as its anchor."

O' Hisham! Everything has its indicator. Man's thoughtfulness is the hall-mark of his reason. The symbol of thoughtfulness is silence. Everything has its climax and the climax of reason is humility (before Allah's commands). It is enough to prove you are ignorant and unwise if you do what Allah has prohibited.

O' Hisham! Almighty God has mentioned about the people of reason in high terms, and has adorned them with the finest ornaments as per His own words:

He gives Wisdom to whomsoever He wills, and whoever is given the Wisdom, has been given much good; yet none will grasp the message but the possessors of understanding minds. [Qur'an 2:269]

And those who rooted firmly in knowledge say: We believe in it; all is from our Lord; yet none remembers, but the men who possess minds. [Qur'an 3:7]

What? Is he who knows what has been sent down to you from your Lord is the truth, like him who is blind (void of knowledge)? Only the possessors of understanding minds will remember. [Qur'an 13:19]

(O' Muhammad!) A book We have sent down to you, blessed, that the people of understanding may ponder its signs and so remember. [Qur'an 38:29]

Imam Musa al-Kazim added:

O' Hisham! God has appointed His prophets and messengers to make people wise and mindful of God. The more they accept and believe in these messengers, the greater is their God-consciousness. The wisest among men is he who knows about Allah most. He who is the most perfect in his reasoning enjoys the highest status among people in this life and the life of hereafter.

O' Hisham! God has given two proofs for the guidance of mankind - the explicit/external, and the implicit/internal. The explicit/external proof is in the form of prophets, messengers, and the Imams (divinely- appointed guides). And the implicit/internal proof is in the form of reasoning and understanding.

O' Hisham! A man of reason does not let his lawful occupations prevent him from thanksgiving/service to Allah, nor does the attraction of unlawful acts wear out his patient.

O' Hisham! One who allows three things to be dominated by another set of three things, has actually destroyed his reason. The first thing is to allow the reason to be dominated by excessive hopes and expectations. The second thing is to allow the highest of wisdom to be dominated by excessive utterances. The third thing is to allow his admonition light to be extinguished/dominated by carnal desires. And the one who destroys his reason (by doing the above three) also destroys both his worldly life and his faith.

O' Hisham! Truth is a sign of obeying God (i.e., truth leads to obey God). And there is NO salvation except through obedience to God. Obedience to Him can only be performed by means of knowledge. And knowledge is a matter of acquisition, and this acquisition is only through reason and intellect. And there can be NO knowledge except through the learned in divinity (`Aalimun Rabbani). And the consciousness of knowledge is through reason and intellect.

O' Hisham! Virtuous conduct of a learned man, though limited, is acceptable (to God) at many times of its value. On the contrary, virtuous conduct of a greedy, lustful, and ignorant person is totally rejected.

O' Hisham! Limited material possessions for a man with sound wisdom are perfectly acceptable to him. Whereas unlimited wealth for a man with little wisdom is not at all accepted to him; and this is why such people have actually earned profit of the trade of their life (in this world). (i.e., they will have no share of profit in hereafter).

O' Hisham! Lo! A man of reason pondered over the world and its people. He realized that he could NOT get the success in worldly life without hard struggle. Then he pondered over the life of hereafter and again realized that the success in the life of hereafter would NOT also be secured without hard struggle. Thereupon, the man of reason decided in favor of the struggle which brought eternal success, as compared with the temporary one of the worldly life.

O' Hisham! The wise people remain detached from the worldly possessions and its attractions, and they remain attached to the life of hereafter. It is because they have realized that the world attracts/lures and is lured; and also, the life of hereafter lures and is lured. But the world itself runs after the man who struggles for the life of hereafter to the extent that he receives in full the share which has been allotted to him for this worldly life. On the contrary, the one who spends his life and struggles for the worldly gains will continue till such time as the life of hereafter demands its own share in the form of death. It is then that he realizes that he is the loser of both, this life and the hereafter.

O' Hisham! Whoever seeks comfort without wealth, and relief free from jealousy, and security for his faith, must implore God from the bottom of his heart for making his reason perfect.

O' Hisham! Almighty God has related about a group of virtuous people who used to pray:

Our Lord! Let not our hearts deviate after You have guided us; and grant us mercy from Your own Presence, for surely You are the Grantor of bounties without measure. [Qur'an 3:8]

In offering this prayer, they realized that human hearts could be deflected and could tend towards blindness and low-ness. Secondly they also understood that those who do not seek guidance and reason from God, are the people who are not afraid of God's (punishments) at all. And whoever is deprived of reason by God, can never be firm and steady in God consciousness in a way as to enable him to realize the truth of God in his own heart. No one can reach this stage unless his deeds corroborate his words... God Almighty never guides the inside of reason, except what comes out of it through deeds and the words.

O' Hisham! The Commander of Believers, Imam Ali used to say: "Of signs of a man of reason, one is that he has got three qualities:

(1) He replies only when he is questioned

(2) speaks when all others fail

(3) Advises what is suitable for the good of the person (who is talking to).

One who has none of these three qualities is stupid."

The Commander of Believers further added: "No one should preside over a meeting except when he possesses either all the three or at least one of the above qualities. If a person presides over a meeting without having all or (at least) one of the above three qualities, he is just stupid!"

O' Hisham! The Commander of Believers (i.e., Imam Ali), used to say:

"Of all the observance and devotions offered to God, the best one is of and by (the people of) reason. Reason of a person does not reach perfection unless it possesses some specific characteristics. The characteristics (of a perfect reason) is that:

(1) It gives immunity against disbelief (Kufr) and evil

(2) Virtue and true guidance is expected to follow from such a man (of reason)

(3) He exhausts his excessive wealth in good deeds

(4) He protects himself from irresponsible gossips

(5) His share in the worldly life amounts to what is needed for his existence

(6) He is never fed up with knowledge throughout his life

(7) Humiliation in pursuit of (belief, obedience and favor of) God is dearer to him than any honor in pursuit of things other than God

(8) Humbleness is dearer to him than dignity

(9) He regards the little good of others to him as much, and regards any good from him to others as little

(10)He regards people better than himself, and regards himself in his heart as of little consequence.

O' Hisham! He who has no generosity, has no religion, and he who has no reason has no generosity. The greatest man is he who never deems himself equal in value to the worldly life. Verily There is NO lower price for your bodies except to be sold in exchange for Paradise. Hence do NOT sell it for anything less than that.

O' Hisham! God has condemned majority as criterion of truth with the words:

And if you obey the majority of those on earth, they will lead you astray from the path of God. [Qur'an 6:116]

And if you ask them: Who created the heavens and the earth? Certainly they will say Allah. Say: All praise belongs to Allah. Nay, but the majority of them have no knowledge. [Qur'an 31:25]

And if you ask them: Who sends down water from heaven thereby revives the earth after it's death? Certainly they will say Allah. Say: All praise be to Allah. Nay, but most of them have no understanding. [Qur'an 29:63]

and He said:

But most of them do not know [Qur'an 6:37, Qur'an 7:131, Qur'an 8:34, Qur'an 10:55, Qur'an 27:61, Qur'an 28:57, Qur'an 39:49, Qur'an 44:39, Qur'an 52:47]

and said:

And most of them have no understanding [Qur'an 5:103]

and said:

But most of them are not thankful [Qur'an 10:60, Qur'an 27:73]

O' Hisham! On the contrary, Allah has praised the minority, by saying:

Few of My servants are thankful. [Qur'an 34:13]

and also said:

And how few they are! [Qur'an 38:24]

and said:

And those who believed, and they did not believed with him (Noah) but a few [Qur'an 11:40]