From Principles to Statutes: Conclusion and Determination

If natural law is general, how do communities get specific laws such as speed limits or tax rates? Aquinas answered that human law derives from natural law in two ways. Some laws follow as conclusions: from 'do no harm' comes 'do not kill', which is binding partly because it expresses natural law itself. Other laws are determinations, like an architect choosing the exact shape of a house within the general idea of a house.

Natural law requires that wrongdoers be punished, but it does not say whether the penalty should be a fine or prison; human lawmakers must decide. This distinction explains why many laws can differ across countries without either being unjust, and why some laws, such as bans on murder, appear almost everywhere. It also limits natural law's ambitions: it does not dictate every rule, leaving wide room for political choice, custom and democratic debate.