Natural law theory claims that there is a natural design and purpose to everything in the world, and this purpose is what is morally good. It has roots in ancient Greek and Roman philosophy. According to natural law theory, there are basic and fundamental moral laws that are based on natural order established by God and apply universally to all people. St. Thomas Aquinas was a major proponent who argued morality is based on humans' natural inclination to fulfill their purpose as set by God. Natural law theory provides a universal moral code but can be criticized for its strong religious foundations and potential for human reasoning to go wrong.