The natural law school was introduced in the Middle Ages by Thomas Aquinas, where he stated that the source of all sources of law comes from eternal law. The eternal law is the ratio of God which regulates all things he created with the aim of being in accordance with their natural properties.

All creatures, whether with ratio or without ratio, have a natural tendency to live according to that law. Humans, who are part of nature thanks to their ratio, have been prepared by their creator with a system of thought and morality so that humans can distinguish between good and bad, strong and weak, which are part of natural law.

In humans there is a system of thought and morality that distinguishes humans from other creatures of God. Other creatures are subject to their nature without choice, while humans are given alternative choices that can express the will of their creator in the form of laws that must be obeyed or take a path influenced by space, time and place.

Man-made laws that are influenced by space, time, and place are called positive laws. If positive law cannot be adjusted to natural law, positive law loses its legal nature which should regulate orderly and safe coexistence, respect each other, maintain rights and obligations and not harm those next to it. All of this is the "idea" of the divine intellect that created everything and guides it to its purpose in accordance with the provisions contained in its revelation as a guide to human life.