The fact that we can identify what is unjust, even without clear standards for justice, is important for developing laws. While trying to get rid of unjust things may not create a completely new legal system, it can help improve existing laws that most people agree on. A test for injustice can guide how laws should be changed to become fairer, although it cannot build an entirely new set of laws from scratch.
Immanuel Kant understood that his idea of the categorical imperative was a way to figure out what is unjust, not a full explanation of justice itself. This idea connects to modern science, where laws of nature are seen as rules that tell us what cannot happen, and their validity is tested by trying to prove them wrong. In both cases, we can only get closer to finding truth or justice by removing what is false or unjust.
Many people mistakenly believe that justice has to be based on clear positive criteria. Legal positivism shows that no such clear criteria exist, but this does not mean that justice is just a matter of personal opinion. We can still develop fair rules based on negative testing, which helps avoid random ideas about justice and encourages careful changes to laws. Legal positivism also challenges classical liberalism, which assumes a more independent understanding of justice.