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"All the money in the world " , in this sentence we talk about money in general , right ? I've read a book that says if we're talking about things in general we do not use "the".
So why "the money" ?

This sentence is not talking about money in general. It is talking about a specific set: "all the money in the world", as if it was a specific quantity you could receive:

Suppose I gave you a box with all the money in the world. How would you choose to distribute it?

Other examples talking about a specific set of money:

Do you still have the money I gave you for your birthday?

The money donated to charities should be included on your tax forms, if you want to take the deduction.

It is possible to talk about money as a concept, in which case you would not use the definite article. Examples of this:

I don't care too much for money, and money can't buy me love -- John Lennon / Paul McCartney

A wise person should have money in their head, but not in their heart. --Jonathan Swift

Money is a terrible master but an excellent servant. --P.T. Barnum

It is possible to say "all money in the world" to reference the global concept of money, rather than a specific quantity.

All money in the world, in whatever form, relies on the collective agreement that it is worth something. Even a brick of gold has no value to a starving man, unless he can exchange it for food.

[Edit] With regard to FumbleFinger's objection: I would claim "money donated to charities" is either a kind of ellipsis, or else refers to a conceptual subset of the concept of money. In my example, it makes little difference whether I'm talking in general about the practice of donating money, or of a specific instance of some quantity donated. The second half of the sentence applies either way.

Conceptual example: Money (which has been generally) donated to charities should be declared.

Ellipsis example: (The specific quantity of) Money (which you have) donated to charities should be declared.

I think a more in-depth exploration is out of the scope of the question, as OP asks only what the definite article means in this context, and not whether the definite article is required.

You have misunderstood what was meant in that book by "in general".

In this sentence, we are speaking of money and water in general terms:

Money is an alternative to barter.

Water is necessary for life.

But here, even though we are speaking of "all", we're still speaking of the thing in particular:

All the water in the watering hole dries up during the summer dry season.

All the money in the world couldn't get me to do that.

I don't know what book told you that, but it's not the truth.¹

Fact is, there are several reasons we might use the definite article. Macmillan (definition 1b) says:

used when you are referring to familiar things that people deal with regularly

I looked up at the ceiling; suddenly all the lights went out

Many familiar adages and expressions use the definite article even though they talk about things in general. Consider:

• The lion is king of the jungle.


• Top of the morning to you!


• It hit me right between the eyes.


• Religion is the opiate of the masses.


• All the news that's fit to print.

¹See what I did there?

Ditto Andrew's excellent answer, let me just add:

Don't get confused by the difference between how something is in the real world, and how it is grammatically.

Yes, in real life, "all the money in the world" is, well, all the money that exists. Logically, you might say that it's the same as "money" as a general concept. But GRAMMATICALLY, "all the money in the world" is NOT all money that exists, it's a specific set of money, namely, the money that is "in the world". While in real life that may be all money, grammatically it is not.

The big, fancy language scholar term for this particular use of the definite article, "the", is "modadic"—the as in the one and only.

From your example, rephrased:

There is only one collection of all money in the one and only world.

Reference:

8. Monadic

("One of a Kind" or "Unique")

The Article

All the money in the world would not make you happy.

We only have one specific world, and all the money in it is very specific.

Having money in the bank is a good thing, if its yours. [non-specific]

And: The money I have in the bank is none of your business. [specific]

Please note: for certain expressions such as money in the bank, a the is used with bank.