"Mistakes" According to The Fundamental Theorem of Poker

It is very important to understand that when we talk about making a mistake according to the Fundamental Theorem of
Poker, we're not necessarily talking about playing badly. We're talking about a very strange kind of mistake - playing differently from the way you would if you could see all your opponents' cards. If I have a royal flush and someone has a king-high straight flush, that player is making a mistake to call me. But a player surely cannot be accused of playing badly by calling or, as is much more likely, raising with a king-high straight flush. Since he doesn't know what I have, he is making a mistake in a different sense of the word.
In advanced poker you are constantly trying to make your opponent or opponents play in a way that would be incorrect if they knew what you had. Anytime they play in the right way on the basis of what you have, you have not gained a thing. According to the Fundamental Theorem of Poker, you play winning poker by playing as closely as possible to the way you would play if you could see all your opponents' cards; and you try to make your opponents play as far away from this Utopian level as possible. The first goal is accomplished mainly by reading hands and players accurately, because the closer you can come to figuring out someone else's hand, the fewer Fundamental Theorem mistakes you will make. The second goal is accomplished by playing deceptively.