The Mu'tazilites [8th-10th centuries] asserted the primacy of reason, and that one's first duty is to engage in reason and, through it, come to know God. They also thought it their duty to understand revelation in a way that comported with reason, so that if something in the Koran seemed inconsistent with reason, it should not be read literally. It should therefore be taken as metaphor or analogy.
The Mu'tazilites held that God Himself is Reason, and that man's reason is a gift from Him so that he can come to know Him through the order of His creation. Abd al-Jabbar, one of the great theologians, made the statement, "It is obligatory for you to carry out what accords with reason."
Not all of this went over well with the more traditional Muslims. Out of this opposition arose another school of theology that came to be known as the Ash'arites [10th century], after its founder al-Ash'ari. They denied, point by point, everything the Mu'tazalites said. They claimed God isn't reason but pure will and power. He can do anything He wantsHe's not restrained or constrained by anything, including His own word. There is no way one can know what is good or evil through reason, but only through revelation.
Al-Ghazali, the great Ash'arite theologian, said that "no obligations flow from reason, but from the Sharia." So nothing you can know through your reason can guide you in your life as to what is good or just. There is no moral philosophy.
The key here is that God does not forbid murder because it is bad; rather, it is bad because He forbids it. He could change His mind tomorrow and demand ritual murder, and no one could gainsay Him, because things are themselves neither good nor evil, but are only made so by God's commands.
In interpreting God's laws, there is a principle in Islamic jurisprudence which states, "Reason is not a legislator." In other words, the only laws that apply to you are the ones God gave you. Reason has no authority or status in creating laws, or even in interpreting them.
The political consequences of such a view are easy to see: If reason is not a legislator, then why have legislatures at all? They have no standing, because reason has no standing.