zinzir
Legend
Silver Level
I like how simple and obvious it is, yet it took someone like David Sklansky to put it into words 
"Every time you play a hand differently from the way you would have played it if you could see all your opponents' cards, they gain; and every time you play your hand the same way you would have played it if you could see all their cards, they lose. Conversely, every time opponents play their hands differently from the way they would have if they could see all your cards, you gain; and every time they play their hands the same way they would have played if they could see all your cards, you lose."
If interested, you can read a little more about The Fundamental Theorem of Poker here:
https://en.wikipedia.org/wiki/Fundamental_theorem_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, they gain; and every time you play your hand the same way you would have played it if you could see all their cards, they lose. Conversely, every time opponents play their hands differently from the way they would have if they could see all your cards, you gain; and every time they play their hands the same way they would have played if they could see all your cards, you lose."
If interested, you can read a little more about The Fundamental Theorem of Poker here:
https://en.wikipedia.org/wiki/Fundamental_theorem_of_poker