no green unicorns

A mental patient told his doctor that he could see invisible green unicorns. Then the doctor asked him how if they were invisible, how he knew that they were green. OK, so the patient said. "He told me that he was green. Also, he has wings... That's my answer." The doctor said. "There are no green unicorns." Right then the patient replied. "Well, maybe he was cyan. It's really hard to know what your true color is when you are invisible in a mirror."