Psalm 1/Notes/Phrasal.V. 4.492371
From Psalms: Layer by Layer
The article on "chaff" is probably generic. "The article of class marks out not a particular single person or thing but a class of persons, things, or qualities that are unique and determined in themselves... It is especially common in comparisons" (IBHS §13.5.1f; see e.g., Gen 19:28; Isa 34:4).