The premise of ontological relativity (or the Quine-Duhem thesis) is that all theories (and the propositions derived from them) of what exists are not sufficiently determined by empirical data (data, sensory-data, evidence); each theory with its interpretation of the evidence is equally justifiable. Thus, the Greek's worldview of Homeric gods is as credible as the physicists' world of electromagnetic waves. This theory in the philosophy of science was first posited by Pierre Duhem and developed later by W. V. Quine.
While ontological relativity does not invalidate the principle of falsifiability first presented by Karl Popper, Popper himself acknowledged that continual ad hoc modification of a theory provides a means for a theory to avoid being falsified. In this respect, the principle of parsimony, or Occam's Razor, plays a role. This principle presupposes that between multiple theories explaining the same phenomenon, the simplest theory--in this case, the one that is least susceptible to continual ad hoc modification--is to be preferred.
W. V. Quine 1968 Ontological Relativity Available at http://www.unlv.edu/Colleges/Liberal_Arts/Philosophy/Ontological%20Relativity.htm