This was the era of Sigmund Freud (1856–1939) and his highly influential concepts of psychoanalysis. His theories, though notably focussed on the conscious and unconscious mind were unquestionably studied and put to use by many contemporary authors and artists, and Hemingway proved no exception.
If someone talks of subconsciousness, I cannot tell whether he means the term topographically -- to indicate something lying in the mind beneath consciousness -- or qualitatively -- to indicate another consciousness, a subterranean one, as it were. He is probably not clear about any of it. The only trustworthy antithesis is between conscious and unconscious. -- Freud
However, in his review of Hemingway's posthumously published novel, Islands in the Stream, Malcolm Cowley questions the degree to which Hemingway was latterly able to rely upon subconscious thought (as was the case in so many other areas of the author's life) : 'The weakness of the book might be that here, as in other works of his later period, Hemingway was unable to make effective use of his subconscious mind. He had always depended on it and often said that a good half of his work was done in the subconscious: "Things have to happen there before they happen on paper."'