We reconsider the microeconomic foundations of financial economics under Knigh-tian Uncertainty. We remove the (implicit) assumption of a common prior and baseour analysis on a common order instead. Economic viability of asset prices and theabsence of arbitrage are equivalent. We show how the different versions of the Effi-cient Market Hypothesis are related to the assumptions one is willing to impose onthe common order. We also obtain a version of the Fundamental Theorem of AssetPricing using the notion ofsublinearpricing measures. Our approach unifies recentversions of the Fundamental Theorem under a common framework.