Testing a theory generally means attempting to disprove the theory, and Popper argues that this is the only way to establish it as empirical or scientific. The history of science shows that theories are constantly being disproved and rewritten, as we look back at theories such as the Earth being the center of the universe, which was accepted as scientific knowledge at the time. The same process can be observed for almost every element of scientific knowledge. Popper would say that this is an example of the unstable foundation of science, of which current theory is simply the high point, but is still made up of this metaphorical swamp of human beings.