Is 'to be seeing things' the same as 'to have visions'?
I did this ngram search to figure out if there's any difference, and it looks like they have a very similar meaning.
(I don't know why the link doesn't work. But if you paste 'I'm seeing things,I have visions' into the ngram search box, you'll get the results.)
'To have visions', though, seems to have more to do with longer events that happen in a different time, either in the future or the past. Also, visions usually have a specific meaning to either the person experiencing them or those who hear about them. They can act as 'messages', so to speak.
'To be seeing things' instead is usually more about nonsensical hallucinations. They are less 'noble' experience, mystically speaking.
I can infer, therefore, that one can belittle someone who's had visions by telling them 'You must have just been seeing things'.
I'd like to know if my understanding is correct.