He could not tell if what he was seeing was real or if it was a 'hallucination'.
He has been having 'hallucinations' due to the medication.
(..) I'll see if I can find out where. Do that. You had any headaches lately? - Not until you showed up. - It will wear off, along with the hallucinations. See you around. Get along. He has a six months' lease. Used up a little more than five and a half months of it. (..)1954 Rear Window