One consequence of the Duhem-Quine thesis is that any theory can be made compatible with any empirical observation by the addition of extra (ad hoc) hypotheses. This is why science uses Occam's Razor; hypotheses without justification are eliminated. This led Karl Popper, to reject naïve falsification in favor of 'survival of the fittest', or most falsifiable, of scientific theories. In Popper's view, any hypothesis that does not make testable predictions is simply not science. Such a hypothesis may be useful or valuable, but it cannot be said to be science. W.V. Quine thought that empirical data are not enough to make a judgment between theories. In this view, a theory can always be made to fit with the available empirical data. However, this does not necessarily imply that all theories are of equal value, because scientists often use guiding principles such as Occam's Razor.