Miracles are certainly not impossible nor can they be negated through scientific arguments. It can never be proved that the causes and agents that are functioning in the world are solely those that we see and know and that other causes which we do not know or whose effects and actions we have not seen nor understood do not exist. It is in this way possible that in one or several members of mankind there can be operating certain causes and agents which bestow upon them a very long life of a thousand or several thousand years. Medicine has not even lost hope of discovering a way to achieve very long life spans. In any case such protests from "peoples of the Book" such as Jews, Christians and Muslims are most strange for they accept the miracles of the prophets of God according to their own sacred scriptures.
The opponents of Shi'ism also protest that, although Shi'ism considers the Imam necessary in order to expound the injunctions and verities of religion and to guide the people, the occultation of the Imam is the negation of this very purpose, for an Imam in occultation who cannot be reached by mankind cannot be in any way beneficial or effective.
The opponents say that if God wills to bring forth an Imam to reform mankind He is able to create him at the necessary moment and does not need to create him thousands of years earlier. In answer it must be said that such people have not really understood the meaning of the Imam, for in the discussion on the imamate it became clear that the duty of the Imam is not only the formal explanation of the religious sciences and exoteric guidance of the people. In the same way that he has the duty of guiding men outwardly, the Imam also bears the function of walayat and the esoteric guidance of men. It is he who directs man's spiritual life and orients the inner aspect of human action toward God. Clearly, his physical presence or absence has no effect in this matter. The Imam watches over men inwardly and is in communion with the soul and spirit of men even if he be hidden from their physical eyes. His existence is always necessary even if the time has not as yet arrived for his outward appearance and the universal reconstruction that he is to bring about.
source : www.tebyan.net
