I told my doctor I kept seeing invisible people.

This joke tickles our funny bone with a clever bit of wordplay, taking advantage of the double meaning of the word "see." When the patient says they're "seeing invisible people," they're clearly describing a medical symptom, perhaps even a hallucination. It's a serious concern that suggests they're experiencing something outside of normal perception.

The punchline, however, pivots sharply, using "see" in a completely different, much more mundane way. The doctor's response, "I just can't see you right now," refers to their availability for an appointment, as in, they're too busy to fit the patient in. The humor springs from this absurd misinterpretation. Instead of addressing the patient's very unusual complaint about invisible people, the doctor treats it like a standard scheduling conflict, creating a delightfully ironic and unexpected twist that highlights the often-impersonal nature of getting a doctor's appointment in a busy world.