From David Sklansky’s The Theory of Poker:

Every time you play a hand differently from the way you would have played it if you could see all your opponents’ cards, you lose; and every time you play your hand the same way you would have played it if you could see their cards, they lose.

An analogy in trading can be made concerning objectivity while holding a position:

Every time you execute a trade that you would not have executed had you been flat, you lose; additionally, every time you refrain from executing a trade that you would have executed had you been flat, you lose. (more…)