Novelist Brian Herbert, Frank Herbert's son and biographer, explained that "Frank Herbert drew parallels, used spectacular metaphors, and extrapolated present conditions into world systems that seem entirely alien at first blush. But close examination reveals they aren't so different from systems we know". He wrote that the invaluable drug melange "represents, among other things, the finite resource of oil". Michaud explained, "Imagine a substance with the combined worldwide value of cocaine and petroleum and you will have some idea of the power of melange." Each chapter of "Dune" begins with an epigraph excerpted from the fictional writings of the character Princess Irulan. In forms such as diary entries, historical commentary, biography, quotations and philosophy, these writings set tone and provide exposition, context, and other details intended by Herbert to enhance understanding of his complex fictional universe and themes.