Before revision: The Third Fundamental Theorem of Asset Pricing allows to apply the Second Fundamental Theorem under very mild regularity conditions regarding the process of asset prices. It has been shown that an economic equilibrium exists if and only if the price process is a martingale. Hence, market efficiency at least requires the absence of weak arbitrage  , but this is not sufficient to establish a situation where asset prices always "fully reflect " some information beyond the price history .
After revision: Market efficiency at least requires the absence of weak arbitrage , but this is not sufficient to establish a situation where asset prices always "fully reflect " some information beyond the price history .