A government organization established in a country to handle public affairs cannot operate by itself. Unless a group of competent and experienced individuals endeavour to maintain and run it, it will not survive and will not provide the people with its services. The same applies to any other organization established in human societies such as cultural and various economic organizations. These organizations will always be dependent upon competent and honest managers; otherwise, they will be liquidated and wound up in a short period. This is a clear fact that can be perceived by a simple deliberation. Many experiences and experiments also substantiate its veracity.
Surely, the organization of the religion of Islam, which could be called the world's largest organization, follows the same principle. It depends upon guardians and directors for its survival and continuity. It always looks for competent individuals to provide the people with Islamic teachings and laws, to carry out its exact provisions in the Islamic society, and to allow no negligence and procrastination in the observance and safeguarding of Islam.
The guardianship of religious matters and the Islamic society is called "Imamah". The guardian and leader is known as the "Imam". Shi'ahs believe that, after the demise of the Holy Prophet (SA), the Imam must be appointed by the Almighty Allah to act as a custodian and guardian of the sciences and commandments of Islam and to lead the people onto the right path. Whoever truly investigates and conducts a thorough study and research on Islamic teachings and fairly uses his judgement will find out that Imamah is one of the unquestionable principles of the holy religion of Islam and that the Almighty Allah stipulates this matter in Qur'anic Ayat introducing the organization of His religion.
As clarified in the discussion of Nubuwwah, the kindness, favour and care of the Creator of the world which He has towards the world of existence calls for His leading every creature towards a specific goal (which is the attainment of ideal perfection). For instance, a fruit- bearing tree is led towards development, blossom, and bearing fruit. Its course of life differs from that of a bird. Similarly, every bird lives in its own special way and pursues its own specific goal. In this manner, every creature is led only towards reaching its specified goal and pursuing the right path and nothing else. Obviously, man is also one of the creatures of Allah and is bound by the same principle of guidance.
It also became clear that since the ideal perfection and prosperity of man is gained through his own choice and free will, the Divine guidance of man, in particular, should be achieved by way of invitation, propagation, and communication of religion and its rules and regulations through the prophets, so that men could not have any plea against this system of the Almighty Allah. "(We sent) messengers as the givers of good news and as warners, so that people should not have a plea against Allah after the (coming of) messengers...(4:165)."
This holy Ayah leads to the same logic that necessitated the appointment of prophets and the establishment of the system of religious invitation. After the demise of the Holy Prophet (SA), who guarded the religion and led the people with his 'Ismah, it is necessary for the Almighty Allah to assign someone similar to him in 'Ismah and ideal attributes (except wahy and Nubuwwah) as a substitute to preserve the Islamic sciences and commandments without any deviation and lead the people. Otherwise, the programme of general guidance will be disrupted and people would have a plea against the system of guidance of Allah.

THE IMAM IS INDISPENSABLE
Due to its error and mistake, men's wisdom cannot enable them to do without the anbiya' of Allah. In like manner, the existence of Islamic 'ulama' among the Ummah and their religious propagation are not sufficient to make people independent of Imams. As clarified earlier, the question is not whether people follow the religion or not, rather the point under discussion is that the religion of Allah must reach the people untouched and without any change, alteration, or mutilation. It is certain that the 'ulama' of Ummah are not infallible and immune from error and sin, no matter how virtuous and pious they are. It is not impossible that Islamic sciences and laws be ruined and changed by them although it might be unintentionally. The best evidence for this matter is the existence of various sects and discrepancies which have crept into Islam.
Therefore, in any case, the existence of the Imam is necessary to safeguard the real Islamic sciences and laws of the religion of Allah, so that people could benefit from his guidance whenever they find the ability to receive it.

THE HOLY PROPHET'S (SA) STATEMENT ABOUT WILAYAH
Describing the Holy Prophet (SA), the Almighty Allah states: "Certainly a Messenger has come to you from among yourselves, grievous to him is your falling into distress, excessively solicitous respecting you, to the mu'minin (he is) compassionate, merciful (9:128)." It can never be believed that the Holy Prophet (SA), who, according to the explicit wording of the Holy Book, Qur'an was the most compassionate and merciful of all to his ummah, would remain silent all his life and overlook about mentioning one of the divine orders which is undoubtedly of highest importance and topmost priority for the Islamic society and which is demanded by wisdom and commonsense.
The Holy Prophet (SA) knew better than anyone else that the organized and enormous institution of Islam was not a matter of ten or twenty years to be supervised by himself. He was aware that Islam is a universal and eternal organization that must direct the human world forever. Thus, the Holy Prophet (SA) predicting the situation of thousands of years after his demise and issued the necessary orders accordingly. The Holy Prophet (SA) was fully aware that Islam is a social organization and that no social organization can exist and survive even for an hour without a guardian and ruler. Therefore, the presence of a guardian is necessary to preserve the Islamic sciences and laws of religion, to manage the affairs of the society, and to lead and guide the people towards happiness in this world and the Hereafter. Thus, how could it be conceived that the Holy Prophet (SA) would overlook the situation after his departure and would show no interest towards it?
As a habit, the Holy Prophet (SA) used to appoint someone to manage the affairs of people in his absence whenever he left Madinah even for a few days to take part in battles or to perform Hajj. Similarly, he also used to appoint governors for cities conquered by the Muslims and used to assign commanders for every division and group that he despatched to the battlefield. Sometimes, the Holy Prophet (SA) even said: "Your commander is such and such a person. If he is killed, then such and such will be his successor and if he is also killed, so and so will act as the commander." With this policy that the Prophet (SA) followed, how can it be believed that the Holy Prophet (SA) might not have designated anyone as his successor at the time of his death?
In short, by looking deeply into the sublime aims of Islam and the holy objective of its dignified bearer, one will undoubtedly acknowledge that the matter of Imamah and wilayah has been resolved and made clear for the Muslims.

APPOINTMENT OF A SUCCESSOR BY THE HOLY PROPHET (SA)
The Holy Prophet (SA) did not content himself with only general remarks concerning Imamah and wilayah of the affairs of Muslims after his demise, rather he explicitly explained the subject of Imamah, along with Tawhid and Nubuwwah, from the very beginning of his call and announced the wilayah and Imamah of 'Ali (AS) for the affairs of the religion and the world as well as for all the affairs of the Muslims.
According to a hadith narrated by Sunni and Shiah on the first day of his call to the people to Islam 'ulama', the Holy Prophet (SA) invited his relatives to gather publicly for a meeting. At this meeting, he explicitly substantiated and established the ministry, wilayah, and succession of the Leader of the Faithful, 'Ali (AS). In the last days of his life, too, in Ghadir Khumm, the Holy Prophet (SA) lifted 'Ali (AS), holding his hand, among the gathering of one hundred and twenty thousand Muslims and stated: "Anyone for whom I am the guardian and leader, this 'Ali (AS) is his guardian and leader as well."
Furthermore, the Holy Prophet (SA) has specified the number, the names, and all the characteristics of Imams and leaders who will be succeeding him. In a well-known hadith narrated by Shi'ah and Sunni 'ulama', the Holy Prophet (SA) has stated: "There are twelve Imams, all of whom are from the Quraysh." According to the famous hadith, the Holy Prophet (SA) said to Jabir al-Ansari: "There are twelve Imams." Then he mentioned their names one by one and told Jabir: "You will meet the fifth Imam. Say my salam (greetings) to him." Besides, the Holy Prophet (SA) h as specifically appointed the Leader of the Faithful, 'Ali (AS), as his successor. Hadrat 'Ali (AS) has also nominated the next Imam as his successor. In the like manner, each Imam has appointed the next Imam to serve as his successor.