Spanish philosopher Francisco Suárez continued in Aquinas’s tradition, arguing that natural law is the foundation of human law. He described how human laws could be unjust, and placed a greater emphasis on individual liberty and freedom. Man-made laws could, in the view of Suárez, be broken in certain cases. For example, power and authority can be conferred on a ruler by the people, but can also be taken away from the ruler if their laws are unjust. No man-made laws should override people’s natural rights to life and liberty. And since the origin of the state’s authority and power is human, it should be secondary to sacred authority.

The University of Salamanca was home to the School of Salamanca, a group of theologians that included Suárez, who sought to connect the ideas of Aquinas with a changing world.

Suárez’s ideas were controversial, since monarchs across northern Europe claimed divine and absolute authority—the so-called “divine right” of kings. Suárez’s conclusions challenged the notion that the ruler was accountable only to God, and not to the Church or to his or her subjects. By distinguishing between different sources of laws—natural, divine, and human—Suárez rejected the mixing of the secular and the sacred, and separated the realms of power. He also introduced the notion of the social contract, proposing that the ruler governs by the consent of the people, who can also legitimately withdraw their consent if the ruler does not respect the demands of natural law.

Suárez made a distinction between international law and natural law, seeing the former as primarily based on custom and positive law rather than universal rules. Today, the distinction between natural law and positive law remains, both in national jurisdictions and international law. The Declaration of Independence and the Constitution refer to natural law, while English common law has been greatly influenced by natural law theories.