Natural law here means a hypothesis. A proven hypothesis means a theory.
Karl Popper tries to say that the best way to prove a hypothesis is to disprove other false hypotheses.
By disproving other hypotheses, we get closer each to time to know that the original hypothesis is more likely to be correct.
If we fail to refute the original hypothesis, we become more certain that it is likely to be a correct hypothesis.
However, some scientists would argue that the best way to know if a hypothesis is correct or not is to directly prove it by experiments.
By eliminating other hypotheses, we cannot be certain that the original hypothesis is actually correct or not.
Therefore, in order to know whether a hypothesis is correct or not, it is better to prove it using, lab testing, experiments and evidence.
For example, in the Meselson and Stahl experiment, results and evidence were obtained directly to prove the semi-conservative theory of replication.
It is important to know whether a hypothesis is testable or not. Elimination of other hypotheses alone is not sufficient to know whether a hypothesis is correct. It is essential to do experiments and give evidence for a hypothesis before we can make it a theory and then a law.