History and Background
The authority of Sharia is drawn from two major and two lesser sources. The first major source is specific guidance laid down in the Qur'an, and the second source is the Sunnah, literally the 'Way', i.e. the way that Muhammad (the Prophet of Islam) lived his life. (The compilation of all what Muhammad said, did, or approved of is called the Hadith.) A lesser source of authority is Qiyas[?], which is the extension by analogy of existing Sharia law to new situations.
Finally Sharia law can be based on ijma, or consensus. Justification for this final approach is drawn from the Hadith where Muhammad states; "My nation cannot agree on an error." The umma[?], or community of Muslims, comes together with each applying his or ijtihad, or independent thought and judgement, to achieve this consensus. The role of ulema, i.e. scholars, is critical, since they are the ones who studies the Islamic law and therefore are the ones with authority to represent it.
The comprehensive nature of Sharia law is due to the belief that the law must provide all that is necessary for a person's spiritual and physical well-being. All possible actions of a Muslim are divided (in principle) into five categories: obligatory, meritorious, permissible, reprehensible, and forbidden. Fundamental to the obligations of every muslim are the Five Pillars of Islam.
In theory, there is no conflict between the process as outlined by Muhammad and very progressive and consultative political movements, e.g. green parties. In fact, the latter even defined Four Pillars of the Green Party, to some degree in imitation of Islam's Five Pillars, and in admiration of the idea of a consensus-driven process of the whole community coming to some well-reasoned conclusion compatible with science and scholarship. In practice, however, there is often incredible tension between conservative, liberal or secular forces:
Practice of Sharia
Most countries of the Middle East and north Africa maintain a dual system of secular courts and religious courts, in which the religious courts mainly regulate marriage and inheritance. Saudi Arabia and Iran maintain only religious courts for all aspects of jurisprudence. Sharia is also used in Sudan, Libya and for a time in modern Afghanistan. Some states in northern Nigeria have reintroduced Sharia courts.
In practice these have most often meant the re-introduction of spectacular and gruesome punishments (such as amputation of one/both hand(s) for theft, or stoning for adultery) without respecting the much tougher rules of evidence and testimony (including the necessity of four eyewitnesses, with women's testimony counting less than that of a man). Such measures are usually introduced to gain support of local ulema who are often community leaders in rural areas. Their examples are not always humane or even reasonable. Muslim scholars tend to agree that Muhammad himself would not run courts along these lines in an otherwise secular society, nor introduce these punishments into societies rich enough to afford prisons and rehabilitation, cohesive enough to prevent those accused from being killed by outraged victims and communities.
Like Jewish law and Christian canon law, Islamic law has no one, set meaning for all time and places. In the hands of moderates, religious law can be moderate, even liberal. In the hands of post-Englightenment readers of philosophy, religious law is relegated to ritual (as opposed to law in a civil sense), or even to just being history. In the hands of zealots, it becomes legally enforced against all people of a faith, and even against all people that come under their control. Islamic law to American Muslims in Boston is a very different thing than Islamic law to religious Muslims in Saudi Arabia, Gaza, or Pakistan. Both are following Islamic law, yet it varies as much as individual Muslims vary. (As is true for Jews and Christians, etc.)