Fundamental Theorem of Poker

The Fundamental Theorem of Poker was laid out in David Sklansky's seminal work The Theory of Poker. It states, on page 17-18:

"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."

It should be noted that when Sklansky talks about gaining and losing here, he's not talking about winning or losing the hand. He's talking about theoretical gains and losses. If you draw to a gutshot in hold'em without the proper odds, you sustain a loss even if you win the pot. You took the worst of it but got lucky.