In what way does Frankl argue that prisoners could retain "the last of the human freedoms?"

While prisoners' strong reactions to the concentration camp could be seen as proof that, as Frankl writes, "man cannot escape the influence of his surroundings," he argues that this is not the case. He writes: "Man can preserve a vestige of spiritual freedom, of independence of mind, even in such terrible conditions of psychic and physical stress." He argues that the "last of the human freedoms" is "to choose one's attitude in any given set of circumstances, to choose one's own way." Further, Frankl writes that this "spiritual freedom" is what "makes life meaningful and purposeful."