David Sklansky (born 1947) is among the world's most famous gamblers. He is also a best-selling author that has influenced an entire generation of poker players with his many books on the subject. By introducing and explaining advanced strategic concepts to the general public, some believe Sklansky revolutionized the game forever, leveling out the playing field and setting free the secrets of poker.

A concept that was first put into words by Sklansky and has since become the foundation of many poker strategies is the Fundamental Theorem of Poker. It states that "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."

Though the theorem above appears in simple English, it is actually based on heavy mathematical reasoning. It encapsulates the nature of poker as a game of decision-making in the face of incomplete information. And though the theorem sounds simple, applying it to different poker situations is a skill that requires a lifetime of practice to master. If the theorem above seems trivial - please read on, as we will attempt to explain its mathematical reasoning.

When playing poker, you are constantly making decisions. These may be analyzed in terms of their 'expected value', which is a statistical term that expresses the average payoff of a certain decision/action if it is repeated many times. You can find the expected value of a given action by multiplying each outcome value by its probability, and then adding them all together. For example, the expected value of throwing a die is 3.5, since that is the average outcome over time - ((1+2+3+4+5+6) / 6).

In poker, the correct decision to make in most situations (certainly in heads-up play) is the one that has the highest expected value over time. Let's explain this: Even if you knew which cards your opponent was holding, you couldn't always make the winning decision every time, since you still may loose to a very unlucky draw. Rather than trying to make the winning decision, you should always be making the decision with the highest expected value, so over time you will win the most money. So even if you loose a particular hand, your decision may have been statistically correct, since given the same situation – and after repeating the same decision – over time you will win more than you lose.

For example, if you're holding a high pair and you know for a fact that your opponent is drawing to a flush, the decision that has the highest expected value for you might be to raise by more than your opponent's pot odds. Even if your opponent hits his draw and you end up losing more money on the river (a flush is higher than a pair, of course), your decision could have been correct. Here's why: By repeating the same tactic, over time you can expect to win more money during the hands when your opponent misses his draw. You have effectively raised the expected value, thus doing the right thing in the long run.

In heads-up situations the following is always true: If you could see your opponent's cards at all times, you could always make decisions that have the highest expected value over time, thus mathematically calculating the 'correct' decision. Fortunately or unfortunately, the reality of poker is that you can't see your opponent's cards. That's exactly what the Fundamental Theorem of Poker is all about – the more times you play your cards as if you could see all, making decisions that give you the highest expected value of profit, the more you can expect to win in the long run. And the less your opponents do the same – the more you win.

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