seeing things - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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]