This book offers the first sustained jurisprudential inquiry into Islamic natural law theory. It introduces readers to competing theories of Islamic natural law theory based on close readings of Islamic legal sources from as early as the ninth and tenth centuries C.E. In popular debates about Islamic law, modern Muslims perpetuate an image of Islamic law as legislated by God, to whom the devout are bound to obey. Reason alone cannot obligate obedience; at most it can confirm or corroborate what is established by source texts endowed with divine authority. This book shows, however, that premodern Sunni Muslim jurists were not so resolute. They asked whether and how reason alone can be the basis for asserting the good and the bad, and thereby obligations and prohibitions of the Shari'a. They theorized about the authority of reason amidst competing theologies of God. For these jurists, nature became the link between the divine will and human reason. Nature is the product of God's creative power. Nature is created by God and reflects his goodness; consequently nature is fused with both fact and value. As a divinely created good, nature can be investigated to reach both empirical and normative conclusions about the good to be pursued. By recasting the Islamic legal tradition in terms of legal philosophy, the book sheds substantial light on an uncharted tradition of natural law theory and offers critical insights into contemporary global debates about Islamic law and reform.