Testing a theory usually means attempting to disprove the theory, and Popper would say that this is the only way to establish it as empirical or scientific. The history of science shows that theories are constantly refuted and rewritten, as exemplified by those that the Earth is the center of the universe, which was then accepted as scientific knowledge. The same process can be observed for almost all scientific knowledge. Popper would say that this is an example of the unstable foundation of science, with current theory simply being the high point, but still consisting of this metaphorical swamp of humans.