What was one of the precautions of trade according to Dr. Dischord?

From the text:

"Of course not," replied the doctor, pouring himself a small glass of the liquid; "that's what makes it so difficult to cure. I only treat illnesses that don't exist: that way, if I can't cure them, there's no harm done—just one of the precautions of the trade," he concluded, and, seeing that no one was about to take his medicine, he again reached toward the shelf, removed a dark-amber bottle, dusted it carefully, and placed it on the table in front of him.