I went to the doctor and told him I kept seeing invisible people.

This joke tickles our funny bone through a clever combination of wordplay and a touch of absurdity. The setup establishes a seemingly serious, albeit fantastical, problem: seeing invisible people. The humor then pivots entirely on the doctor's perfectly logical, yet entirely unexpected, solution. The punchline, "He told me to stop taking imaginary medicine," brilliantly flips the script, suggesting the patient's delusion is self-induced by an equally non-existent remedy.

The brilliance here lies in the circular logic. Instead of offering a complex diagnosis or treatment for hallucinations, the doctor's advice directly mirrors the patient's complaint, implying the "invisible people" are a direct, and comically fitting, side effect of "imaginary medicine." It's a playful twist on cause and effect, where the remedy is as unreal as the affliction, making us chuckle at the sheer simplicity and wit of the doctor's deduction. This kind of humor often pokes fun at the idea of self-inflicted problems, a common theme in comedic scenarios involving doctor visits.