Sklansky: What's the odds on you helping me with the odds?

The Theory of Poker, page 36, 2nd parargraph.

Figuring the odds for making a hand is done on the basis of the number of unseen cards and the number among them that will make the hand........

Here is my question, unless you are playing poker by yourself, then at least 5 of those "unseen" cards (the ones dealt to your opponent) have been removed from the deck.
Leaving only 42 cards instead of 47. Keep in mind this is
assuming you have only one opponent. If there are only 42 cards left in the deck how can you base you calculation on 47 cards? I just do not understnad why he is using unseen cards instead of the actual number of cards left in the deck.

In addition how do correct you calculation for the possiblility that some of your hand making cards may have been dealt to your ooponents? For example if you hold 4 to a flush, how can you assume that there are 9 cards (9 of the same suit) that are left to make your hand? If five
of your flush cards have been dealt to your opponents than you have a serious miscalculation of your odds of making the flush. I do not understand how the calculation of the odds can be correct when it is based on an assumtion which can be wrong.

The correct calculation of odds, I would think, would go as follows.


Actual number of cards left in the deck / True number of cards left in the deck that will make you hand

Please help.

Thanks