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Good_Burning_Plastic comments on Open thread, Aug. 10 - Aug. 16, 2015 - Less Wrong Discussion
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If the Efficient Market Hypothesis is true, shouldn't it be almost as hard to lose money on the market as it is to gain money? Let's say you had a strategy S that reliably loses money. Shouldn't you be able to define an inverse strategy S', that buys when S sells and sells when S buys, that reliably earns money? For the sake of argument rule out obvious errors like offering to buy a stock for $1 more than its current price.
The EMH works because everybody is trying to gain money, so everybody except you trying to gain money and you trying to lose money isn't the symmetric situation. The symmetric situation is everybody trying to lose money, in which case it'd be pretty hard indeed to do so. And if everybody except you was trying to lose money and you were trying to gain money it'd be pretty easy for you to do so. I think this would also be the case in absence of taxes and transaction costs. IOW I think Viliam nailed it and other people got red herrings.