What is the philosophy behind Al-Mahdi's (peace be upon him) existence?

When Islam endorses common consciousness and stresses that ultimately this world will be filled with justice and equity after having been filled with injustice and oppression, it gives a factual value to this hope and creates a definite belief in the future course of humanity. This belief is not merely a source of consolation, but it is also a source of virtue and strength. It is a source of virtue because the belief in the Mahdi (peace be upon him) means the total elimination of injustice and oppression prevailing in the world. It is a source of inexhaustible strength because it provides hope which enables man to resist frustration, however, hopeless and dismal the circumstances may appear to be. This belief in the appointed day proves that it is possible for the forces of justice to face the world.

filled with injustice and oppression, to prevail upon the forces of injustice and to reconstruct the world order. Prevalence of injustice, however dominant and extensive it may become, is an abnormal state and must in the long run be eliminated. The prospect of eliminating it after it has reached its climax gives hope to every persecuted individual and every oppressed nation.

Although the concept of the Mahdi (peace be upon him) is more widespread than the Muslim community, its detailed features, as revealed by Islam, more fully meet all the aspirations attached to this hope since the dawn of history. They are in greater conformity with the feelings and sentiments of the oppressed and the persecuted of all times. It is no longer necessary to look forward to an unknown savior who may come into the world in the distant future. Rather, Islam tells us that this savior is already here and we simply have to look for the day when the circumstances are ripe for him to appear and begin his great mission. The Mahdi (peace be upon him) is no longer an idea. He is no longer a prophecy. We need not wait for his birth. He already exists and we are only waiting for the inauguration of his role. He is living amongst us in his real human griefs. He witnessed all the acts of oppression and persecution which are perpetrated on the face of the earth and, somehow or other, he himself is affected by them. He is anxiously awaiting the moment when he will be able to extend his helping hand to everyone to whom any wrong has been done and be able to eradicate injustice and oppression completely.
It is evident that the concept of the Mahdi (peace be upon him), with its Islamic features, shortens the gap between the oppressed and the expected Savior. It spans the bridge between them, however long the period of waiting may be.

We are asked to believe that the Mahdi (peace be upon him) is a particular person already living a normal life. We are also expected to believe that the idea of absolute eradication of every kind of injustice and oppression by the Mahdi (peace be upon him) has already been embodied in the person of the Awaited Savior. He will reappear when he will be, as the narration says, "owing no allegiance to any tyrant."

This tradition urges the believers in the Mahdi (peace be upon him) to keep waiting for him and to continue looking forward to solace. The idea is to establish a close spiritual and intuitive link between the believers, on the one hand, and the Mahdi (peace be upon him) and all that he stands for, on the other. It is not possible to establish such a link without believing that the Mahdi (peace be upon him) has already been born and is a living and contemporary personality.

Thus we find that the concept of the living Mahdi (peace be upon him) has given a new impetus to the idea of an expected Savior. It has made it a source of effective strength and consolation to every person suffering from deprivation and injustice, a person who rejects all forms of tyranny because he feels that his Leader -being a contemporary and a living personality and not a future idea- shares his sufferings and feels his misery.