I have been thinking quite a bit about Islam. One difference, I think, boils down to this: Catholic Christian thinkers eventually reached a consensus that natural law must be derived, at least to some significant degree, from the nature of man—that is derived through reason, from the very nature of God’s creature or God’s creation. While there was no unanimity, Catholic thought—which WAS western philosophy because theology and philosophy were not split, but rather were merged essentially from the beginning of Christendom until perhaps about 1600 years after Christ—eventually moved in this general direction.
It was not always clear that the Church would move in this direction. Certainly Duns Scotus and William of Occam and others tended to see natural law as something imposed from above by the wording of scripture. But they lost out. Aquinas and the Spanish scholastic Francis Suarez were important in this debate and indeed Gabriel Vasquez, who was a Spanish contemporary of Suarez, viewed the rational nature of man as the primary source of natural law and man’s obligation to obey the natural law. You move from that base easily along to Grotius, Pufendorf and Locke, and the idea of rights, freedom, and so forth.
Islam clearly did not follow that road. Law was a product of the word of God—certainly this is how I understand Sharia. While there are other factors—e.g. Christ’s standing outside the state, being neither a ruler nor a military man, and Mohammed being within the state as it were, a ruler and a military man—certainly the view of law is an important distinction. Islam, I think, views law as something that is imposed from the top down, that is they believe in something consistent with legal positivism; law is set out by the authority, and it must be followed or punishment follows. Whereas Catholic thought laid the critically important groundwork for the notion that law comes—in some significant way—from the bottom up; that is that the rules that should govern society come from the nature of man as derived by human reason, thereby placing reason in the forefront of Catholic political and legal thought.
— M.S., Canadian lawyer, friend