1
general::
Phrase(s): seeing things
imagining that one sees someone or something that is not there. • Lisa says that she saw a ghost, but she was just seeing things. • I thought I was seeing things when Bill walked into the room. Someone had told me he was dead.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Experiencing hallucinations or delusions, as in I thought I saw my father, but I must have been seeing things; he died twenty years ago. [First half of 1900s]
American Heritage Idioms