Why is it idiomatic to say the mirror instead of a mirror?

Obviously when you examine your reflection you could use any mirror, so "a" should be expected... right?

  • 9
    Look in a mirror is also idiomatic, but it means a random mirror, not your mirror, the one you habitually use for grooming. That's the mirror. Feb 7, 2021 at 18:52
  • 3
    If you're serious, the answer is that you can sponsor a sociolinguistic survey. Other than that, it's all impressions and anecdotes, with random ideas of what counts as "right". Feb 7, 2021 at 18:54
  • 1
    Does this answer your question? Definite article — “on television” vs. “on the radio” Feb 7, 2021 at 19:16
  • 2
    Are you guys sure this isn't an explored and labelled linguistic phenomenon? You are all treating it so trivially but it is super interesting to me
    – minseong
    Feb 7, 2021 at 19:21
  • 3
    It doesn't become a possessive; it merely becomes definite. Similarly, the wife, the toothache, the grippe, the flu, the libido -- who they belong to is part of what they mean. Things that are intensely personal to start with become distinctively so when they become definite. Feb 7, 2021 at 19:31

4 Answers 4


“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.

  • 3
    does this usage of "the" have a name?
    – minseong
    Feb 8, 2021 at 14:40
  • 1
    I don't see this as a generic usage. 'The mirror has / mirrors have been used since antiquity' show generic usages. 'In the mirror' seems more akin to 'off the wall' or 'off the cuff' where there is phrasal coherence. We could say 'Look at yourself in a concave mirror' but not 'look at yourself in the concave mirror' if one wasn't around. But we can speak generically: 'The concave mirror was invented by Thomas Jefferson'. Feb 8, 2021 at 19:24

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.

  • 4
    Nice one, Anton. Conversely if A sees a grooming error in B's look A might suggest that B "Take a look in a mirror" since any mirror will show the error. Once B has looked in a specific mirror (perhaps the one above the third sink from the left in the first floor toilet) then B can say "I looked in the mirror and I've cleaned the smudge off my cheek now."
    – BoldBen
    Feb 8, 2021 at 1:43
  • Is this phrase used only when the person is actually facing a mirror?
    – jsw29
    Feb 8, 2021 at 22:37
  • @jsw29 Only? Given the diversity of language, that seems so restrictive as to be improbable. I doubt that it is only used in such a circumstance; perhaps it may be used metaphorically too.
    – Anton
    Feb 8, 2021 at 22:44
  • @Anton, indeed, the question was rhetorical; its point was that this answer does not explain the use of the definite article when the person is not facing a mirror.
    – jsw29
    Feb 8, 2021 at 22:49

I can't begin to think about how to research this notion, but . . .

How magical a mirror is! It's like an oracle that one consults: The Mirror. Capital T, capital M

Here's an example sentence from the OED at mirror, n. II. A reflective surface, and related senses 4. a. :

1766   J. FORDYCE Serm. Young Women II. viii. 88   Next morning the mirror is consulted again.
Source: Oxford English Dictionary (login required)

Plus, this magical thing can be in any number of places at once, such are its powers! You can consult it upstairs, you can consult it downstairs, I can consult it next door.

Like the mercury once used to make mirrors, and as in a thermometer when dropped: many silvery balls but still only one mercury, always reflecting the gazer.

Consult a mirror cracked into a thousand pieces, and you'll see a thousand of yourself. The moon sees many ponds, but the ponds see only one moon.

Food for thought: The History of Mirror: Through A Glass, Darkly

PS: I have not started drinking yet today.


In Merriam-Webster's definition for "the", there are many senses given. "Looking in the mirror" is sense 1(l), which reads

used as a function word before the name of a commodity or any familiar appurtenance of daily life to indicate reference to the individual thing, part, or supply thought of as at hand

So the idea is that it is as if the mirror has been specified earlier in the conversation, because there is assumed to be typically at most one relevant mirror at hand in whatever situation the person is in. The standard example of this usage, before personal cell phones became universal, was "called her on the phone."

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.