The intellect is one of the greatest gifts of God to man and with which man has been blessed. God says in the Holy Qur’an:
(It means:) O Prophet, declare that it is God Who has brought you out of non-being and given you existence and bestowed upon you hearing, vision and an aware heart (so that you should thank Him for His bounties), though there are few persons who are grateful to their Lord for His gifts. (67:23)
The Master of the Faithful, may peace be upon him, says:
The intellect is man’s most precious asset, for it restores his dignity after his humiliation, uplifts him if he falls, guides him if he is lost and gives firmness and rectitude to his speech when he speaks. 
In Islamic teachings the intellect has been considered as man’s ‘inner’ prophet and guide and as God’s ‘proof’. Imam al-Kazim, may peace be upon him, said:
God has appointed two kinds of guides for mankind. One is outward and manifest and the other is inward and hidden. The manifest proof are the prophets, messengers and the holy leaders of the faith. The hidden proof is the intellect.
Since the intellectual capacities of people are not of the same level and are different in degrees, every man on the Day of Judgement will be held responsible in proportion to his intellectual ability. The Fifth Imam said:
On the Day of Judgement God shall scrutinise the record of His creatures’ conduct (with a severity) in proportion to their intellectual capacities in the world. 
In the present era man has been greatly fascinated by the wonderful accomplishments of reason in the form of scientific discoveries, considering them the ultimate purpose and end of life. This enchantment has dealt an irremediable blow to the role of the intellect and its place in human life. It has caused him to ignore and neglect the power and faculty which has a direct
connection with the supra-sensible and the Source of being. Had the enchanted man seen the distant and wider horizon and ventured into the vast panorama of the supra-sensible, he would not have stopped at the fascinating manifestations of reason. Islam is fully cognisant of the real worth and capacity of the intellect and the scope of its activity. It is on the basis of this knowledge that it has given so much care to the training and growth of the intellect so that it may view the realities of existence with thoughtful care. The Qur’an asks the intellect not to hold on to anything which has not been proved with
certainty and beyond doubt. It requires the intellect not to accept anything until there is a clear and decisive proof to justify such acceptance.
And follow not that which thou hast no knowledge of; the hearing, the sight, the heart-all of those shall be questioned of. (17:36)
This express warning clearly underlines the necessity of making sufficient investigation before accepting anything as true. Similarly, the Qur’an points out the deviant character of those who do not base their beliefs on certainty and merely follow their conjectures and presumptions. It says about them:
They follow only surmise, and surmise avails naught against truth. (53:28)
Thereupon by adopting a firm approach based on an unshakeable reasoning it demolishes the intellectual foundations of blind imitation and surmise. It warns the blind and unquestioning followers who blindly imitate the creed and beliefs of their ancestors that their approach is sheer folly.
They say, ‘No; but we will follow such things as we found our fathers doing.’ What? And if their fathers had no understanding of anything, if they were not guided ? (2:170)
These exhortations are aimed to develop a critical mind and to put the intellect back in its true role by rejecting reliance on surmise and conjecture. By this means it seeks to habituate the intellect to discipline and critical scrutiny in its field of action, so that thereby it may regulate the various faculties and set in order the ideas and conceptions under its dominion. The kind of thinking that Islam requires is not one of an abstract kind removed from concrete realities that takes the form of philosophical speculation. By calling attention to the signs (ayat) of creation it seeks to awaken the intellect so that man may
employ his conscious faculties to contemplate profoundly regarding the signs of Divine Majesty and Wisdom manifested in the system of creation. It is a thinking that is free from fantasy, free and perceptive of realities, not one which is lost in the dark wilderness of fancies. It is thinking that links man, with his perception and senses, to the Divine Spirit that circulates through the entire world of being, and this is the highest merit of the intellect.
Spinoza, the European philosopher, writes:
The highest thing which the mind can understand is God, that is to say, Being absolutely infinite, and without whom nothing can be nor can be conceived, and therefore that which is chiefly profitable to the mind, or which is the highest good of the mind, is the knowledge of God. Again, the mind acts only in so far as it understands and only in so far can it be absolutely said to act in conformity with virtue. To understand, therefore, is the absolute virtue of the mind. But the highest thing which the mind can understand is God (as we have already demonstrated), and therefore the highest virtue of the mind is to understand or know God. 
The ultimate purpose of thought and inquiry in Islam is to cure the human heart and to lay the foundation of life on truth and justice. When a person arrives at a certain conclusion through thinking and is made profoundly conscious of its implications, he puts it into action and implements it in his practical life. Once that dynamic conviction informs his thought, behaviour and faculties of perception, he is prepared for a serious struggle against every indignity that compromises the real worth of the human being. Although the intellect is the best guide and the biggest source of discernment, it loses its brilliance as a
result of the curtain that obfuscating desires and lusts draw over the intellect and obstruct its light. Then the intellect practically loses its capacity of guidance. The Qur’an refers to the misguiding role of desires and lusts in these
Then (O Prophet) if they do not respond to thee, know that they are only following their desires: and who is further astray than he who follows his desire without guidance from God? Surely God guides not the evildoers. (28:50)
Nay, but the evildoers follow their own desires, without knowledge…. (30:29)
Had the Truth followed their desires, the heavens and the earth and whosoever is in them had surely corrupted. (23:71)
Hast thou seen him who has taken his desire to be his god, and God has led him astray out of a knowledge. (45:23)
Without doubt, to overcome and control one’s desires and negative urges is a very difficult task. Only with persisting efforts and exercises can one keep the rebellious passions in check and make them tractable and subject to reason. This is the way to overcome the tyranny of desires and to benefit from them in a right and worthy manner. The Noble Prophet, may God bless him and his Household, once addressing a group of warriors returning from the battlefield said to them:
‘Bravo to those who have accomplished the minor jihad and who yet remain to wage the major jihad!’ He was asked, ‘O Messenger of Allah, what is that major jihad?’ He replied, ‘That is the jihad against the self.
He who attains bliss and nearness to God is one who is vigilant over his violent and dangerous urges and one who does not allow his carnal motives to dominate his intellect and turn him to impiety and aberration.
But as for him who fears the station of his Lord and forbids the soul its desire, surely Paradise shall be the refuge. (79:40-41)
References:.Al-Amidi, Ghurar al-hikam, p. 212. . Al-Kulayni, al-Kafi, vol. 1, p. 16. . Ibid., "kitab al-‘aql wa al-jahl." . Spinoza, Ethics, in Man and Spirit: The Speculative Philosophers; ed. by Saxe Commins and Robert N. Linscott, p. 175. . Al-Saduq, Ma’ani al-akhbar, p. 160.
Source: Selected extract from Ethics and Spiritual Growth by Sayyed Mujtaba Musawi Lari