The Doctor and the Medical Texbooks

One doctor was poor in medical skill and he had never cured any patients.His friend scolded him,saying,"don't you know how to cure the sickness at all?"
"No, you are wrong,my friend,"said the doctor,"I know quite well about my skill.I cure the sickness completely according to the medical textbooks.It is my patients who are to blame not I,because they never come down with illness as the books say."