I have questions, both general and specific, on the definite article "the."
He grew up in New York as the son of Australian immigrants, the oldest of three boys.
In the sentence above, there are three sons in the family, and the person in question is the oldest of them. But he is described as the son of Australian immigrants instead of a son. I wonder what kind of logic is at play.
I first thought, that if someone is the son of, say, an American husband and a British wife, then they have only one son. But I have found, according to my English usage book, they may have actually more than one son, something counterintuitive to me given the underlining logic of "the"--"you know which one I mean"--that implies there should have been no son other than the son in question that the reader can only identify.
The section "the son of Australian immigrants, the oldest of three boys" in the sentence above, is even more peculiar to me because the writer makes it clear that there are three boys, but still uses the definite article "the" in front of "son of Australian immigrants."
Why and how do native speakers use "the" when it is impossible for the reader/listener to tell which person(s) or thing(s) in several similar possibilities?
In the first place, what is the difference between the two sentences below?
(1) He is the son of Australian immigrants.
(2) He is a son of Australian immigrants.
My usage book says that sentence (1) may or may not have more than one son, then is there any difference between the two sentences? Does "a" son in sentence (2) strongly suggest the possibility of more than one son?
In a similar vein, such phrases as "the wrong answer" are often used even though there are apparently numerous possible wrong answers, but "the wrong answer" seems to be more natural and standard than "a wrong answer"
Please explain these uses of the definite article "the" when the reader/listener has no means or it is impossible to specify which one(s) is/are the writer/speaker is discussing?