's Fundamental Theorem
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.
Sklansky makes the assumption that the poker player knows what to do when all of his or her opponents cards are known. This involves calculating the probabilities that each hand has to win and calculating pot odds.
The idea here is that the real goal of poker is to play as if you have perfect information and that each time you make a mistake in guessing your opponent's cards, you lose a slight edge. Likewise, you can gain an edge by deceiving your opponent into making a move he or she wouldn't, if all the cards were known (i.e. successfully bluffing).
Gaining an edge in poker is a matter of consistently making decisions that are likely to make you money. And, in the long run, after many hands -- due to the law of large numbers -- the randomness of the cards becomes irrelevant.