English teacher here. Articles in English can be really confusing but essentially we use the definite article in this situation because:
Uniqueness: In most situations, there's only one mirror in a room or a home that's readily available for someone to look into.
Generality: Similar to "going to the bathroom," "look in the mirror" refers to the general act of using a mirror to see oneself, not interacting with any specific mirror.
Ukrainian here. IMO, the first statement is half-stupid, the second one is half-overcomplicated :) no offense to you personally, of course. I understand the whole concept of articles in English and know (at least I thought I knew before this post) their correct usage, and in all use cases I can remember the article uses are logically acceptable for a foreigner, but this one with the mirror and the bathroom is messed up a bit :)
The second example is quite good imo. You would never say “I’m going to a bathroom.” Even if you were in a stadium with hundreds of bathrooms, you would still say you are going to the bathroom.
Same as you might say I’m taking the train. Not usually taking a train, though I’ve heard that too sometimes. Though oddly you usually say I’m taking a plane, not the plane. Also I’m taking the freeway, not a freeway. I’m usually going to the doctor, less often than a doctor.
Tell me you haven't read Jonathan Strange without telling me you haven't read Jonathan Strange 😏 obviously it's because all mirrors are connected - as entrances to the King's Way of old.
My assumption would be that it's because we don't really look at mirrors per se but rather the reflection in them, so the definite article is indicating the fungibility of the mirror itself. This total speculation on my part though and I might be totally wrong.
Maybe there's a cultural idea about mirrors being somehow "the same". After all, a mirror shows the same thing regardless of which one it is. Or related in cultural mythology to a singular adjoining world that contains your doppelganger (in such media, you don't usually have a separate mirror-self for every mirror, but one that can be accessed from any mirror). Also could be a turn of phrase that stuck without a good reason.
We say "the doctor" when talking about the concept of a doctor. We tend say "my doctor" and not "the doctor" when talking about what our respective doctor told us. Kind of like how we refer to the clock as "my clock" when we notice a difference to the universally accepted concept.
I suspect it has to do with being a sort of household appliance. Similar to the fridge, the TV, the bathtub, etc. People think about it in that sense most frequently and it becomes the common parlance.
Take these sentences where a similar thing happens.
"Look out the window."
"I'm heading to the gym."
"You should hold the door for people."
"You need a trip to the barber."
In these cases, the noun isn't actually that important, more than it is what you're doing with the noun. These nouns represent the general act of doing something, and I guess since that action is a singular specific thing, we use "the".
This applies to "Look in the mirror." The actual mirror doesn't really matter much. The focus is on the general act of looking at your clear reflection.
Yes I know this, the reason that I asked this question is because it is a departure from the rules laid out here. Oftentimes we say “the mirror” even though we are not referring to any specific mirror.
Mirrors used to be expensive so I imagine it came from a whole family sharing just 1. And perhaps they were not common enough for them to even think about other mirrors. So they would just refer to the singular mirror they had.
Do you think maybe 'a mirror' refers to actually visually looking at a mirror and 'the mirror' refers to taking inventory of yourself? Unless there's actually a mirror nearby that you're referring to.