Why did natives use 'a' over 'the' in the 2 following cases?

Question

Recently I asked a question about articles. Since I did not understand that case clearly I continued on this topic and have a couple of another questions why authors (native English speakers) used 'a' over 'the'.

1. A person says:

Next summer, I'll be on a island, with your girlfriend, and a pocket full of numbers.

Why did he say 'a pocket' if it's obvious he was talking about his pocket?

2. Another person says:

I'm riding with a friend that is high as me

Why did he say 'a friend' if the situation occurred in the car in which there were nobody except them 2.

P.S. Actually, the words 'girlfriend' and 'friend' were replaced over strong language.

Answer 1

There's two types of articles - definite (the) and indefinite (a/an). There's also not using an article, sometimes called the zero article.

What does it mean for X definite or indefinite? Definite in this case simply means:

• "which X" matters,

• the speaker/writer expects the listener/reader to know which X if asked.

Indefinite means:

• "which X" does not matter

Because this depends on a common understanding between speaker/writer and listener/reader, or an assumed understanding, you often can't tell whether something is definite or not merely from the words in the sentence.

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Next summer, I'll be on an island, with your girlfriend, and a pocket full of numbers.

You don't need to care which island the speaker/writer will be on, or which pocket the numbers are in.

Next summer, I'll be on the island, with your girlfriend, and a pocket full of numbers.

Speaker/writer expects you to know which island. If you didn't actually know, you'd have to ask and give away your ignorance.

I'm riding with a friend that is high as me

You don't need to care which friend is riding with the speaker/writer. This may be because the speaker/writer doesn't want to tell you or believes you don't know his/her friend.

Answer 2

Because the words “pocket” and “friend” are just introduced — they were not already used before.

It doesn't matter that we may precisely determine which pocket/friend it is, the only important thing is that they are just introduced, and they are not names of unique objects (as e.g. the Earth).

“I have an apple. The apple is red.” (After introducing an object, we use “the”.)

Answer 3

Besides MarianD's remarks: a pocket full of numbers may suggest a measurement – a pocketful, a collection of numbers (women's telephone numbers?) that suffices to fill up a pocket – rather than the contents of a literal pocket.