In late 1938, Whorf's own health declined. After an operation for cancer he fell into an unproductive period. He was also deeply influenced by Sapir's death in early 1939. It was in the writings of his last two years that he laid out the research program of linguistic relativity. His 1939 memorial article for Sapir, "The Relation of Habitual Thought And Behavior to Language", in particular has been taken to be Whorf's definitive statement of the issue, and is his most frequently quoted piece.