The Fundamental Theorem of Poker is a principle of poker as written by David Sklansky. The truth of the theorem can be demonstrated through numerous examples with particular consideration to expected value. It heavily implies what skills and considerations required to play a winning game 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."

The theorem from the book The Theory of Poker by David Sklansky.