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It is said that when we are referring to a noun for the first time then we use an indefinite article.
For example,
"I disagree with the thesis written by Guya Jones.
Here thesis is mentioned for the first time then why is 'the' used instead of 'a'?
Take another example,
There is a book in my bag.
Here 'a' sounds perfectly OK.
Why is the article 'the' used in the former sentence?
These uses involve the difference between predicates and modifiers.
A predicate is some 'new' fact your sentence asserts about its subject. The entire assertion, including both subject and predicate, is called a 'predication'.)
In your There is a book in my bag is a sort of paraphrase of the predication
A book is in my bag.
A book, designating some indefinite (unspecified) book, is the subject of the sentence, and (is) in my bag is the predicate. This predication does not specify what book you are talking about, it only asserts the presence of some indefinite (unspecified) book in your bag.
In subsequent sentences, however, you may treat this book as definite (specified)—
The book in my bag is red.
Now in my bag is no longer a predicate but . . .
A modifier. A modifier is something 'presupposed' about its head, the noun it modifies; it's an 'old' fact.
And if the modifier identifies its head sufficiently for your hearer to know exactly which instance of 'noun' you are talking about, the noun phrase is definite (specified) and may take the definite article, *even if 'noun' has not been mentioned previously.
Note, however, that you are not required to treat a modified noun as definite. You use the definite article to signal your hearer or reader that you are speaking of a specific instance of 'noun' which you expect her to be familiar with and to recognize.
We all know articles are very difficult for language learners because there are so many exceptions. But I find the following general rules go a long way to cover many of the uses. For the question that was submitted, I think Rule 2 gives an idea of why 'the' was used:
Rule 1: Indefinite articles (a, an) are used for previously unknown nouns that are being introduced into a dialogue or story and definite articles are used for nouns that have already been introduced (or are already known or are assumed to be known at the point of introduction to the conversation).
For example: I saw a cat. The cat was sitting on a fence. The fence was painted brown. The cat jumped off the fence when it saw a mouse. The mouse ran into a hole when it saw the cat so the cat didn't catch the mouse.
Rule 2: The definite article 'the' is used when something is unique or, in other words, there is only one of that object.
For example: The sun, the president, the queen of England, the capital city, and the moon
It is used for objects that are well known by many or most people, but it is true even when the hearer may not know the object: A:Who's he? B:He's the president of Korea. She's the CFO. He's the mayor.
This can be contrasted with: A: Who's she? B: She's a member of parliament. She's an accountant. He's an alderman.
This uniqueness can come by association: A car crashed into a tree. The driver was seriously injured.
Rule 3: When we are speaking of a noun in general we usually leave the article out, and, if it is countable, use the plural form.*
For Example: I like cookies. This is talking about cookies in general. However, if I say 'I like the chocolate chip cookies.' I am talking about specific cookies.