In accordance with the sound disposition and the pure nature that are innate in him, man is ceaselessly engaged in the struggle to develop and advance towards perfection.
Consciously or unconsciously, with a love that quickens his spirit, he moves forward in the direction of the utmost dignity and nobility man can attain. This is a reality that is always manifest in humans; spiritual need impels them to advance ever further along their path in order to approach higher degrees and more exalted values. This evolutionary process passes through various degrees which are firmly and profoundly interlinked.
It is of course true that within man unbridled and unholy desires exist that are hostile to this enterprise, and throughout the course of his forward motion man must constantly battle against the destructive inner forces that threaten to rob him of his powers and sacrifice him to forces of evil.
As long as man exists on the plane of being, this struggle towards perfection will also exist. Its aim and culmination must be clear, and there must exist also in human society an exceptional individual who thanks to his spiritual qualities has penetrated to the inner meaning of all laws, a personage who while fully engaged in the struggle has never once fallen prey to deviation.
Such an individual or personage is what is intended by the term Imam. He is the truly liberated man, the chosen herald of monotheism; in his exalted person all conceivable have been realized and rendered active.
As the vanguard of the humanity, he is the divinely appointed link and intermediary between the world of the unseen and the human race. Without himself needing any intermediary, he is guided directly by God. Like a lamp burning in the heart of the darkness, through the teachings that have come to him from heaven, he enables everyone to rise and ascend to the degree permitted by his spiritual ability and capacity. He employs his intelligence, his faith and his will in order to impel them forward to the most exalted degrees and to guide them to the superabundant source of unity, justice and purity.
Were human society to lack such a divinely chosen person, man would be unable by the efforts of his intellect alone to find his directions, no link would exist between the human race and the world of the unseen, and man's efforts to attain perfection would falter and fail.
It is inconceivable that after equipping man with the urge to attain perfection and bestowing on him the potentiality of ascent to lofty degree, God would not lay before him the path leading there or deprive him of the guide that he needs.
On the contrary, God's infinite grace necessitates that He should demonstrate to man the path for attaining the truths of religion and assist him by placing before him a comprehensive scheme ensuring his welfare in this world and his eternal bliss in the hereafter. This comprehensive scheme, embracing all dimensions of human existence, is precisely what God has conveyed to mankind by means of His chosen messengers.
According to the creed of monotheism, none but God can rule over the created universe. In the world of man, which is but a part of the universe, sovereignty must similarly belong to God. It is true that within the sphere of his acts man has freedom of choice, based on the free will that has been allotted to him, but in order for him to harmonize himself with the universe of which he is a part, he must act in accordance with God's commands and refrain from encroaching on His sovereignty. If he fails to respect the laws brought by the prophets, a disharmony and lack of concordance between mall and the universe will arise, and he will inevitably find himself deviating from his intended course.
In just the same way that obedience to revealed law and to the Prophet who may be regarded as the quintessence of all the monotheistic movements in history is the same as obedience to God, the one who wishes to rule monotheistic society as the successor of the Prophet must possess the same inner attributes of communication with God; only then will obedience to him accord with man's purposive advance.
From the time that the Messenger of God, peace and blessings be upon him and his family, founded the government of the righteous and prepared the way for the creation of a pure and luminous society, he undertook also the educative programs he had elaborated. However, since the life of the Prophet was transitory like that of other men, it was necessary that as soon as that great educator had departed, a successor should come to the fore, a righteous and worthy man possessing all the attributes needed to lead the Muslims, who would continue the directive and educative role exercised by the Prophet, in the most desirable or even ideal form.
Embodying all the qualities of a perfect human being, he nurtures the spirits of his followers by means of his superabundant spirituality, and he shows them the way of advancing along God's path towards God, obeying God's commands and turning away from all other than God. Only thus will the straight path remain open, enabling everyone to embark on the road to felicity.
We will understand all of this better once we realize that there is no line of demarcation between this world and the hereafter, and that regulations pertaining to man's bodily life cannot be separated from laws relating to his spiritual existence; a specific guardian has been chosen for both. For this reason the pure and inerrant one chosen by God must gather in his hands the reins of the affairs of both this world and the hereafter, and guard the general and universal interest of Islam against other peoples and nations.
Through the blessed existence of this true leader, this representative of God upon earth, the sole path that exists for attaining true happiness remain open before men. With his spiritual richness and wisdom of conduct, he guides them on the road at the end of which they will find, in the presence of God, all the pure and noble qualities for which they yearn.
It is true that among the Twelve Imams it was only 'Ali b. Abi Talib who exercised rule, and that for a limited period. The other Imams never possessed governmental powers, and they were not permitted to use the position of leadership that was rightfully theirs to strengthen the position of the Qur'an, to expand the culture of Islam, or to develop the identity of the ummah. But this was the fault of the people, who failed to make it possible for them to assume power and as a result were deprived of the benefits that might have accrued to them from these unparalleled exemplars of mankind. For in appointing the Imams, God had established His proof before men; He had presented them with these righteous and exceptional men, chosen ones whose existence was a source of benefit not only for all Muslims but for all of mankind.
In addition to this, it is important to remember that the beneficial effects of the existence of the Imams were not limited to their exercise of political power; they fulfilled their appointed mission in a variety of other ways. The Imam was responsible for preserving the very truth of religion and for keeping God's religion unsullied by distortion and manipulation. Both God and the Messenger had given him the task of instructing people in the verities of the Qur'an and the teachings of religion, thus giving proper direction to their lives.
Moreover, the Imam is a channel for God's grace, so that even if people were deprived of the government of justice and equity that the inerrant Imams would have created thanks to their own incapacity and lethargy they did benefit from the other dimensions of the Imams' existence and activity. They were the channels of God's grace irrespective of whether or not they were permitted to rule and lead Islamic society. Superabundant virtue flowed forth from their beings, bringing men's potentialities to fruition.
The preservation of the very foundations of religion was intimately connected with the attention paid to the subject by the Imams, for awareness of their presence among the ummah was able to prevent many basic deviations from taking place.
Like an alert and careful observer, 'Ali b. Abi Talib, peace be upon him, followed all that was taking place in his time.
Whenever an incorrect verdict was issued, a law was distorted, or an incorrect penalty was about to be applied, 'Ali looked into the matter and gave the necessary instructions, He was stringent and honest in protecting the principles and laws of Islam.
He exercised leadership at all stages of his life. Thus he was always prepared to answer the scholars of other religions who came flocked to Madinah in order to put their queries before the legatee of the Prophet, peace and blessings be upon him and his family.
It was thanks to the blessed existence of the Imam that Islamic learning - the legal, educational, and social teachings of the faith were disseminated among the Muslims and the vital commands and ordinances of the Qur'an became widely known. Even in lands ruled by harsh and savage rulers, at a time when the caliphs were sunk in corruption and transgression and strove to prevent society from imbibing Islamic knowledge, the numerous utterances and traditions of the Imams, rich in learning and wisdom and pertaining to all aspects of the faith, served to preserve religion and give the necessary guidance to society.
Some of the caliphs like al-Ma'mun sought to destroy the scholarly credentials of the Imams by arranging debates and disputations among the scholars of different religions and sects, but the performance of the Imams in these gatherings served only to reinforce their scholarly prestige.
The Imams, as heirs to the teachings of the Messenger, bequeathed thousands of hadith to the scholars of Islam, hadith that originated on various occasions and had the purpose of enlightening society on religious matters and clarifying the credal bases of the faith. They pertained to all the different concerns of jurisprudence, to ethics and moral conduct, and to esoteric knowledge. It was by drawing on these resources that scholars were able to disseminate the Islamic sciences widely in society and to elaborate an authentic jurisprudence as opposed to the various legal currents then in existence.
We will be better able to appreciate the incomparable struggle waged by the Imams in the service of Islamic culture in all of its branches if we compare the hadith of the Sunnis with the traditions narrated from the inerrant Imams. This comparison will demonstrate the profundity of vision, the originality of thought, and the varied knowledge of the headers of Shi'ism. The Sunni scholars themselves have benefited to some degree from the knowledge and learning of the Shi'i Imams, for consciously or unconsciously they have borrowed a great deal from them in this respect. The Imams thus vindicated fully their function as the true guardians of Islam.
It was Imam Ja'far al-Sadiq, peace be upon him, who introduced philosophy, theology, mathematics and chemistry for the first time. Among his companions, al-Mufaddah b. 'Umar, Mu'min al-Taq, Hisham b. Hakam, and Hisham b. Salim were specialists in philosophy and theology. Jabir b. Hayyan specialized in mathematics and chemistry, and Zararah, Muhammad b. Muslim, Jamil b. Darraj, Hamran b. A'yan, Abu Basir, and 'Abdullah b. Sinan, in jurisprudence (fiqh), principhes of jurisprudence (usul al-fiqh) and Qur'anic exegesis. 
Ibn Shahrashub writes:
"From no one have so many traditions been narrated as from Imam Ja'far al-Sadiq, peace be Upon him. As many as four thousand students derived their knowledge from him, and some of the founders of the Sunni schools of law also drew on that storehouse of learning." 
Among his students were the founders of law schools (madhahib) such as Malik b. Anas, Sufyan al-Thawri, Ibn 'Uyaynah, and Abu Hanifah; jurists (fuqaha') such as Muhammad b. Hasan al-Shaybani and Yahya b. Sa'id; and traditionists (muhaddithin) such as Ayyub al-Sijistani, Shu'bah b. al-Hajjaj, and Abd al-Malik b. Jurayh. 
Ibn Abi 'l-Hadid, who is regarded as a great scholar among the Sunnis, writes the following concerning the genial character of 'Ali b. Abi Talib, peace be upon him:
"What can I say of a person to whom all human virtues have been attributed? Every group counts him as one of their own; every virtue arises from his being; and every science and branch of learning goes back to him. Theosophy, the most noble of all forms of knowledge, is derived from his utterances. The teacher of Wasil b. 'Ata' who was the leader of the Mu'tazilah, benefited from the instruction of 'Ali by two intermediate generations. Likewise, whatever learning the Ash'arites have, they also owe to 'Ali.
"Without any doubt, the philosophy and theology of the Shi'is and the Zaydis also go back to 'Ali. He is the supreme teacher of all jurists, for Abu Hanifah, the founder of the Hanafi school, was a pupil of Imam Ja'far al-Sadiq, peace be upon him, who had imbibed the learning of 'Ali through transmission by his father and ancestors. Malik b. Anas, the founder of Maliki jurisprudence, had a master who was the pupil of 'Ikrimah, who in turn had been a pupil of Ibn Abbas, who had derived his learnirng directly from 'Ali.
"'Umar b. al-Khattab would always turn to 'Ali for help in solving difficult questions, and he would often say: 'Were it not for 'Ali, 'Umar would be lost.'
"As for the jurisprudence of the Shi'ah, it goes without saying that it goes back to their first leader. In addition, 'Ali was the master teacher of all exegetes of the Qur'an. This can be easily ascertained by referring to the books of exegesis and seeing how most of their material springs from him. Even that which is narrated from Ibn 'Abbas ultimately goes back to 'Ali. Ibn Abbas was once asked: 'How would you compare your knowledge with that of your cousin?' He replied: 'Mine is like a drop, and his like an ocean.'
"All the great gnostics ('urafa') attach themselves to 'Ali, and he is in addition the one who founded the science of grammar, having taught its fundamental principles for the first time to Abu 'l-Aswad." 
 Asad Haydar, al-Imam al-Sadiq
wa al-Madhahib al-Arba'
 Ibn Shahrashub, al-Manaqib,
Vol. IV, p.247.
 Asad Haydar, Imam Sadiq wa
Madhahib-i Chaharganeh, (Persian translation), Vol. III, 27-28,46.
 Ibn Abi 'l-Hadid, Sharh, Vol. I, p.6.