Leadership of Imāms
The fourth principle of the primary principles of the religion is the leadership of Imāms.
Imāmat signifies the leadership and management of the mundane and religious affairs of the people and the succession of the Prophet of Islam, Muḥammad ibn ‘Abdullāh (pbuh). The necessity of the succession of Imāms after the Prophet is inevitable on two grounds:
1. Social life won’t survive without a leader whose remarks and edicts have profound influence. Until the Prophet was living, he was in charge of leading and ruling Muslims. It should be borne in mind that the Prophet is both a heavenly chosen messenger and the leader and ruler of Muslims.
Therefore, after the demise of the Prophet (pbuh), his successor must immediately assume the leadership to pursue the objectives of the Prophet, that is, to spread monotheism across the globe, and lead the people to the desired end. Otherwise, the efforts of the great Prophet (pbuh) would be voided and Muslims would be detained on their journey to perfection.
2. The great Prophet besides being the ruler and leader of Muslims, was also a divine instructor and a heavenly messenger who explained the facts to people. After the demise of the Prophet, if there were no Imām or leader who would explicate religious facts and concepts and thus carry on with the mission of the Prophet, this would constitute a shortcoming and defect in the Islamic world. Therefore, it is vitally significant that the Imām who is the successor of the Prophet, immediately assume the management of the affairs and fulfill the needs of people in terms of religious and faith-related questions. And the meaning of the statement that the Prophet has explained the faith of Islam completely is that he has outlined the general points completely. Thus, the explication of these general points, and the elimination of ambiguities and the instructions of the lofty facts of Koran certainly necessitate a great instructor whose knowledge is on a par with that of the Prophet.
Now, I draw your attention to the following debate conducted between one of the students of Imām aṣ-Ṣādiq (‘a) and a Sunnī scientist, on the necessity of Imāms and leaders after the Prophet.
Hishām, one of the youngest and most prominent students of Imām al-Ṣādiq (‘a) says, “On a Friday, I arrived in the city of Baṣrah, and went to the mosque. ‘Amr ibn ‘Ubayd Mu’taḍidī was at the mosque and a large crowd had huddled around him asking questions.
I went up and sat among them. Everyone was asking questions. I turned toward ‘Amr and said, ‘O the great scholar! I am not from this city; may I ask you a question?’
‘Amr said, ‘ask what you like.’
I said, ‘Do you have eyes?’
He said, ‘Don’t you see I have eyes; why do you ask?’
I said, ‘My questions are of this type.’
He said, ‘Ask though they are useless.’
‘Do you have eyes?’
‘What do you do with your eyes?’
‘I look at beautiful things, and distinguish between colors and types.’
‘Do you have a tongue?’
‘What do you do with that?’
‘With that I taste the flavor of food.’
‘Do you have nose and the power of smelling?’
What do you do with that?’
‘With that I smell the odors and distinguish between pleasing and disgusting odors.’
‘Do you have ears?’
‘What do you do with them?’
‘With them I hear the sounds and distinguish between them.’
‘Do you have heart (intelligence) besides these?’
‘What do you do with that?’
‘If other parts are doubtful, heart will put an end to their hesitation. Because it is evident that sensual perceptions are occasionally erroneous and to rectify their mistakes, I consult the court of my intelligence and heart.’
Hishām says, “As ‘Amr ibn ‘Ubayd finished there, I confirmed all his remarks and said, ‘Indeed, God, the Exalted, has created the heart to lead the senses and rectify their errors. O great sage! Does it make sense to contend that God who has not left eyes, ears and other parts without a guide, such beneficent God could have abandoned Muslims without a guide and leader after the demise of the Prophet, so that people continue to plunge into doubts and divisions and finally into destruction and annihilation? Could it be accepted by common sense?’
Hishām says, ‘When I drew such a significant conclusion from a series of simple questions, ‘Amr ibn ‘Ubayd understood that I am a Shī‘ah and one of Imām aṣ-Ṣādiq’s (‘a) disciples, therefore he fell silent and could not answer me and by beating around the bush he concluded the debate.’”
Hishām intended to draw this conclusion from this debate that God has definitely appointed some leaders as the successors of the Prophet who will lead Muslims one after another.
Indeed, as in the words of Avicenna, the famous Muslim philosopher, “Does it sound logical that a God who has provided man with eyebrows and eyelashes and has been so solicitous in meeting the needs of mankind, to neglect his most important need, that of guidance, and abandon mankind without a guide and leader and instructor and desert people?”
 ‘Uṣūl al-Kāfī, vol. 1, p. 170.