Friedrich Nietzsche referred to the "World Riddle" in his Also sprach Zarathustra (Thus Spoke Zarathustra) written during 1883–1885; however, his direct influence was limited to a few years, by his failing health. Although Nietzsche had become a professor at age 25, Nietzsche left due to illness at age 34 with a pension in 1879, became an independent philosopher for only ten years, then spent his final 11 years in asylums.
View of Haeckel
Ernst Haeckel viewed the World Riddle as a dual-question of the form, "What is the nature of the physical universe and what is the nature of human thinking?" which he explained would have a single answer since humans and the universe were contained within one system, a mono-system, as Haeckel wrote in 1895: [4][5]
[From Monism as Connecting Religion and Science by Ernst Haeckel (translated):]
"The following lecture on Monism is an informal address delivered extemporaneously on October 9, 1892, at Altenburg, on the seventy-fifth anniversary of the "Naturforschende Gesellschaft des Osterlandes." ... The "exacting" Berlinphysiologist shut this knowledge out from his mind, and, with a short-sightedness almost inconceivable, placed this special neurological question alongside of the one great "world-riddle," the fundamental question of substance, the general question of the connection between matter and energy. As I long ago pointed out, these two great questions are not two separate "world-riddles." The neurological problem of consciousness is only a special case of the all-comprehending cosmological problem, the question of substance. "If we understood the nature of matter and energy, we should also understand how the substance underlying them can under certain conditions feel, desire, and think." Consciousness, like feeling and willing, among the higher animals is a mechanical work of the ganglion-cells, and as such must be carried back to chemical and physical events in the plasma of these. -Ernst Haeckel, 1895[5]
Haeckel had written that human behavior and feeling could be explained, within the laws of the physical universe, as "mechanical work of the ganglion-cells" as stated.
"All the great single-word answers to the world's riddle, such as God, the One, Reason, Law, Spirit, Matter, Nature, Polarity, the Dialectic Process, the Idea, the Self, the Oversoul, draw the admiration that men have lavished on them from this oracular role. By amateurs in philosophy and professionals alike, the universe is represented as a queer sort of petrified sphinx whose appeal to man consists in a monotonous challenge to his divining powers. THE Truth: what a perfect idol of the rationalistic mind!"
William James has questioned the attitude of thinking that a single answer applies to everything or everyone. In the passage, the capitalized "THE" signifies the viewpoint meaning "the one and only" absolute truth.
^ ab "Biography of Ernst Heinrich Haeckel, 1834–1919" (article), Missouri Association for Creation, Inc., based on 1911 Britannica, webpage: Gennet-Haeckel: life, career & beliefs.
^ "Classic Records Catalog / LSC-1806: Liner Notes" (description), Chicago Symphony Orchestra, R. D. Darrell, Radio Corporation of America (RCA), 1960, webpage: CSO-AlsoSprach.
^ ab "7mono10 txt" (description of Ernst Haeckel's book Monism as Connecting Religion and Science), Project Gutenberg Literary Archive Foundation, Gutenberg.org webpage: GutenbergOrg-7mono10: book "translated from German by J. Gilchrist, M.A., B.Sc., PH.D."].
^ "The Project Gutenberg EBook of Pragmatism, by William James" (text), Project Gutenberg, 2002, Gutenberg.org webpage: Gutenberg-Pragmatism.
Ernst Haeckel, Monism as Connecting Religion and Science ("translated from German by J. Gilchrist, M.A., B.Sc., PH.D."), Project Gutenberg Literary Archive Foundation, Gutenberg.org webpage: GutenbergOrg-7mono10 (for free download).