The efficient-market hypothesis (EMH)[a] is a hypothesis in financial economics that states that asset prices reflect all available information. A direct implication is that it is impossible to "beat the market" consistently on a risk-adjusted basis since market prices should only react to new information.
If you believe in EMH, it would seem difficult for the market to ever be surprised, right? We know the market is constantly surprised, but this is mainly the result of non-public information eventually being disseminated to all players at the same time. The Information Age has allowed even novice investors to receive public information as quickly as the experts. So, if I told you I had material “public” (opposite of non public) information on Apple that is being widely ignored, you should scoff at me, correct? It should not be possible for Apple’s stock to be at an all time high price and P/E (recent history) and for me to believe business is very weak based on public statements by its suppliers, right? I must be wrong.