To see visual hallucinations or imagine things that are not really there. There's no dog in the back yard—you're seeing things. I could have sworn the contract said we were entitled to a lower price, but I must have been 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.
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]