The author was writing in the time of Osler, the 20th century's greatest physician, who revolutionised the art of practicing medicine systematically at the bedside. Clearly things were different then! In those days it really was possible for a doctor to know everything there was to know about medicine, and we knew nothing about the fallibility of the human brain and cognitive errors. But the list of why physicians err in 1915 probably still applies today ... don't you think?
Michael Bliss. William Osler - a life in medicine. Oxford University Press, 1999.