be seeing things
To be seeing visual hallucinations or imagining things that are not really there. Typically used in a figurative sense. 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.
be seeing things
be hallucinating. 1987 Rohinton Mistry Tales from Firozsha Baag How much fun they made of me. Calling me crazy, saying it is time for old ayah to go back to Goa…she is seeing things.