The political systems are based on certain theoretical foundations. One of these theories which is called natural theory believing that establishment of political society is rooted in instinct and nature of human beings. In this regard, Aristotle explains that the first natural state is old age village with its natural order automatically shaped on the basis of instinctive demand. Aristotle believes that human beings are created differently and so they can play different roles in social and political lives. Those who own stronger rationality are proper for social leadership, but those who have weaker talent are appropriate for the rest of social roles and should naturally formed the rest of social elements. However, he did not introduce any fair mechanism for identification of such natural distinctions and neither believed in human equality. Some of these Greek philosophers likened the political leader of human society to head that does the job of thinking, the military forces to chest or feet that is responsible for defending body, and the economic workers to hands that are accountable for economic tasks.  Objectively, Aristotle believes that the main task of the government is to correct the moral defects of the people and also instill carnal virtues in human beings.
The next theory which explains the philosophy of establishing political society is divine theory defining different basis for the formation of a political system. It believes that political order which results from creation of the state and government originate from divine wisdom and will. This theory is attributed to religious scholars and philosophers but they did not initiate any acceptable model or mechanism to be applicable in modern era. Their major goal of establishing political society is human weal, justice and promotion of ethics with sacred attitude towards politics and governance. Based on this, even the executors of politics is considered sacred. The market for such a theory was very hot during the Middle Ages in the West and the most famous theorists of this type of theory were St. Augustine and other church fathers. Abu Ali Sina Balkhi, Ghazali, and dozens of other Islamic thinkers have been exponents of this theory in Islamic society.
Although the divine theory follows a sacred goal, in practical steps they faced challenges. They provide no mechanism, no realistic guarantee to reach those goals and no pluralistic approach to be acceptable for all. The next major problem of this theory is giving no role or position to people in political system. Based on this theory, the people do not know what are their goodness and badness and they should blindly show their satisfaction against what is decided by the top leaders. In fact, people have no rights to ask any why questions while the religion and religious trustees should be the main directors of the political system in society. On the basis of this theory that continued more than a thousand years in the middle Ages the popes were able to take over the institution of government exclusively.
In addition to the two above theories, there are some other theories including evolution theory, domination theory, economic theory and social contract theory for the formation of political institutions. Though discussion about all is beyond the limit of this article, what is needed here is to be discussed is social contract theory. Based on this theory, the political institution was formed when the natural state of human life was disrupted and tended to a new form as we call it social contract.  In modern age, the institution of politics and government system such as republican is the product of this theory. For example, the constitution is a social contract document drawing the foundation political system and role of people in a political system. So, the republican political system is clear example of social contracts which paves the way for public participation and enables people to decide about their future and political destiny.
In republican system, the public participations are ensured through formation of political organizations, political parties, associations, civil society, mass media and the opposition. People can criticize the government and government leaders and even their representatives can summon the government officials for interrogation about their performances. In fact, the mechanisms of republican system resemble a very beautiful scene of human political interactions.  Therefore, the republican system is desirable for people of Afghanistan and they paid very heavy price for establishment and protection of this system in last two decades. 
As pointed out, In order to build a republican system it is necessary to create a collective agreement under the title of the constitution with the direct and indirect intervention of the people. The people should select their representatives so that they can play role in their collective destiny in different political procedures including drafting constitution itself. The constitution is one of the foundations of the republican system, which is created by competent, knowledgeable and committed representatives of the people so as to propel the system in accordance with public interests. The political structures are another matter must be tailored to the needs of people. The structure that promises dynamism and reduction of public problems in certain social contect must be defined as a principle.
In general, the republican system has the capacity that let the people play key roles in formation of government organization and participation.  For example, election is a mechanism that let the people participate in a political society. 
Accordingly, voting and candidacy are the rights of everyone to seek power peacefully, not through force or other authoritarian tricks. In fact, the legitimacy in republic political system comes from public vote and satisfaction. Although the legitimacy commenced with public vote, it continued with public satisfactions.  Based on this, the republican leaders are legitimate as long as they satisfactorily serve or else their legitimacy are atomically declined even though they do not complete the usual period of four or five years.