There is however a difference between a plumber and a doctor: when you see a doctor, your life is potentially on the line, which means that a doctor has much greater responsibilities.

A plumber might choose his or her trade exclusively to make money, but to be a doctor is a greater calling which implies greater care and responsibilities, and those who cannot assume such responsibilities because they lack in honesty or skills should choose another profession, such as perhaps working full time as sales people for the pharmaceutical industry.