Why "the" in "looking in the mirror"?
“The” has several functions in English. Some of them are:
- to reference a previously-mentioned noun: there was a dog; the dog was happy
- to specify: the middle hat
- to elevate the status of a noun: the dessert (with “the” stressed and pronounced as thee without a leading vowel sound in the next word)
- to reference a generic noun: the student isn’t greater than the teacher
Your quote sounds like the generic function of the definite article if it is set in a context like, “When looking in the mirror, you see yourself.” Although there is no specific mirror in view, using the places an emphasis on “mirror” as the focus of the sentence. Consider the following:
- When looking in the mirror, you see yourself.
- When looking in a mirror, you see the self.
There is a subtle change in emphasis that follows the noun with the definite article.
I rather like this question because it queries an often used idiom that I have never seen queried previously.
My simple answer is that at the moment of looking in a mirror there is only one mirror in question - the mirror that I am facing. All others are irrelevant to what is happening. Being unique, the mirror in which I look merits the definite article rather than the indefinite article.