Natural law is the theory that there is a moral law that is inherent in human nature and is knowable by reason. This theory was first proposed by Aristotle, but was later elaborated on by Thomas Aquinas. Aquinas argued that natural law is based on the nature of God and the nature of humans. He said that God is the source of all good, and that human beings are created in the image of God. Therefore, natural law is based on the goodness of God and the nature of humans. Aquinas also argued that natural law is binding on all people, regardless of whether they believe in God or not.