The ideal reasoner, he remarked, would, when he had once been shown a single fact in all its bearings, deduce from it not only all the chain of events which led up to it but also all the results which would follow from it.

He had maintained a platonic relationship with Jean while his first wife was still alive out of loyalty to her. A sequel to A Study in Scarlet was commissioned and The Sign of the Four appeared in Lippincott's Magazine in February 1890 under agreement with the Ward Lock company. Conan Doyle fathered five children.

He was a prolific writer whose other works include fantasy and science fiction stories plays romances poetry non-fiction and historical novels.