Dr Campbell Clark

In the short story The Fourth Man, Dr Campbell Clark is a "physician and mental specialist" (a psychiatrist) who finds himself sharing a train compartment with three other men during the events in the story. Clark himself is very successful in his profession and his book The Problem of the Unconscious Mind was "the most discussed book" the previous year.

At the time of the events in the story, Clark is on his way to Newcastle to treat a case of a dual personality. This leads to a discussion between with two other occupants of the compartment, Canon Parfitt and Sir George Durand on the topic. In Clark's opinion, and using the analogy of the human body as a house, why could there not be two or more residents? He then gives the others background information on a celebrated case of multiple personalities: that of a woman named Felicie Bault who had committed suicide recently and who appeared to have strangled herself.

Durand, a lawyer, thought that the three of them, with expertise in the legal, religious and medical fields, would cover the ground fairly comprehensively and from all the relevant angles when discussing the topic. However Clark disagreed. There remains the opinion of "the man in the street". In his experience, "the man in the street" often had "the thing that all expert opinion must lack—the personal point of view." In the end a knowledge of personal relationships always proves critical. These words would prove prophetic as all along the three man had ignore a fourth man in the compartment who appeared to be asleep.

Campbell Clark is described as a man with a square jaw, very steady blue eyes and reddish hair "untouched by grey, but thinning rapidly". He gave the impression of someone with "a very forceful personality".