Why did St. Thomas Aquinas believe societies need laws in addition to natural law?

St. Thomas Aquinas believed that societies require human-enacted laws (positive law) in addition to natural law because law serves as a "promulgated plan of coordination" necessary to achieve the common good of a political community.1

While natural law provides objective moral principles, Aquinas argued that positive law is essential for two primary reasons:

• Immediate Deduction: Some laws are directly derived from moral principles, such as the prohibition of homicide based on the moral truth that murder is wrong.1
• Determination (determinatio): Many practical aspects of social life require specific rules to facilitate coordination where morality does not dictate a single outcome. For example, while morality requires that people avoid harming others, it does not specify whether one must drive on the left or right side of the road. Once a legitimate authority determines a rule to ensure safety and order, that law becomes binding.1

Ultimately, Aquinas viewed law as an "ordinance of reason" designed to help society realize tangible and intangible goods that could not be achieved through individual action alone.1