Neither explicitly. I would seek more information. I would expect to be caught up in a The Leftovers type situation (2% of the world's population inexplicably vanishes in an instant). That is to say, I would not expect to be the only one affected.

Either way, I would also seek evidence this person ever existed. If I'm truly the only one who remembers them, maybe the problem is with me. Logically speaking, I mean. There is a school of thought called "Last Thursdayism" that says that everything before last Thursday (or some abstract point in the near past) was invented by our brains. The break between short-term and long-term memory. The film Dark City plays upon this concept. ("Does anyone know how to get to Shell Beach?")

Past a certain point, I'd be very careful who I speak to about something that seemingly I am the only person who believes differently from the rest of society.

I'm the only one remembering? Yeah I'm visiting a head doctor

I’d start looking for what filled their place: who graduated in their place? What person is filling their job position? Their brothers/sisters are the same? Wife/husband and kids? Etc.

If there is evidence of a filled void, I’d trust my memory; if not, I’d think I’ve gone crazy.

I would assume it's psychosis and let everyone know I'm going crazy and need supervision. That seems way more likely that someone magically disappearing.

Lost my mind

you’re the only one that have memories of the differences.

In that case I'd believe I hallucinated it. Because the alternative explanation would be a massive conspiracy theory and those are generally silly.

I would never stop until i figured out what happened.

Probably go crazy

I already don't trust my memories.

The answer is: it doesn't matter. It was real to us.

If it is a person that I know in real life, then I always trust my own brain. No doubt. No insecurity.

I would look for psychiatric help and go over memories of that person with other people. I'd probably wonder if they were real or would also disappear.

Edit: oh shit i just remembered that i confuse things that happened in dreams with real life things sometimes. maybe this would be a more extreme version of that? dreaming a whole person. wow

I can't imagine you could hallucinate a person in such detail and over such a long period of time that there would never be any indication they weren't imaginary. I would trust my memories and question the circumstances of their disappearance.

That already happened to me with everyone that I went to school with.

There we graduated and they vanished. Doesn't bother me a bit.

I have the rule of never worrying about things I cannot change. So I would just move on with my life and trust that the person I was and will be is making the best decisions possible given the information and constraints of the time. YOLO

In modern America this is not a hypothetical situation.

Assuming there's nothing wrong with you, there must be something wrong with the universe

It would depend on the person for me. Like, I've got family I abstractly care about, but they're not a very present part of my life, and they could probably vanish without me even noticing.

Then on the other end, completely removing all trace of my partner would be very noticible and distressing.

I need to watch the 4400 again

Well ive had the dream where I had a wife (different person to who im with) kids, a job, full life kinda stuff for like the last 10 years and was really happy. Woke up and it all faded away. Made me sad but carried on with my day. So yeah i guess I'd probably just move on. Also just to clarify my life now is pretty good, so not like I lost anything really.

This is the movie Yesterday, where a failed musician gets knocked out and wakes up in a world where the Beatles never existed. A few other things too, but mainly the Beatles. Spoiler: he becomes a mega star.

Reminds me of the The Lamp Story from ptooey Reddit. https://www.reddit.com/r/AskReddit/comments/oc7rc/comment/c3g4ot3/?context=3

This reminds me of one of my favorite episodes of Star Trek: The Next Generation, "Remember Me

I would assume it was Superboy Prime again

I'd assume I were trapped in a warp bubble.

They go over this in Doctor Who. There's this special light that, when it touches you, it erases you from all time. Everyone forgets you exist (well, supposedly. There's an exception). Good sequence.

Depends. Friend or Pseudofriend or Pseudoenemy from School: I wouldn't notice, except that some subtle records like contacts and photos of them would be gone, but I rarely actually look at those. Person from University: I would think to ask in about 7 months, then be confused no one knows him. Oscar Wilde: I would try rereading The Importance of Being Ernest at some point, the cover not being a very notable feature of my bookshelf, so I wouldn't notice earlier. The guy from TheClick: I watch his content about once every 2 months. Dean Herbert: Let's hope the writers retcon in some other guy who makes a circle clicking video game.

I can honestly say I don't know. I would totally have to be in the situation.