What is the test for a doctor to be qualified as a public professional?

This same principle is expanded beyond the stricture or limitation of a medical practitioner to embrace all public professions in the judgment in Harmer v. Cornelius (1858), 5 C.B.N.S. 236, 141 E.R. 94, the headnote of which reads as follows: "The public profession of an art is a representation and undertaking to all the world that the professor possesses the requisite skill and ability. — When a skilled labourer, artizan, or artist is employed, there is on his part an implied warranty that he is of skill reasonably competent to the task he undertakes." (The italics are mine.)