The term "world riddle" or "world-riddle" has been associated, for over 100 years, with Friedrich Nietzsche (who mentioned Welträthsel in several of his writings) and with the biologist-philosopher Ernst Haeckel, who, as a professor of zoology at the University of Jena, wrote the book Die Welträthsel in 1895–1899, in modern spelling Die Welträtsel (German "The World-riddles"), with the English version published under the title The Riddle of the Universe, 1901.
The term "world riddle" concerns the nature of the universe and the meaning of life.
The question and answer of the World Riddle has also been examined as an inspiration or allegorical meaning within some musical compositions, such as the unresolved harmonic progression at the end of Also sprach Zarathustra (1896) by composer Richard Strauss, made famous in the film 2001: A Space Odyssey.
View of Nietzsche
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:  
- [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" Berlin physiologist 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 
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.
View of William James
The philosopher and psychologist William James has questioned the attitude of thinking that a single answer applies to everything or everyone. In his book Pragmatism (1907) he satirized the world-riddle as follows:
- [From Pragmatism (Lecture VII) by William James:]
- "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, Pragmatism, 1907.
Emil du Bois-Reymond
Emil du Bois-Reymond used the term "World Riddle" in 1880 for seven great questions of science, such as the ultimate nature of matter and the origin of simple sensations. In the lecture to the Berlin Academy of Sciences he declared that neither science nor philosophy could ever explain these riddles.
- Epistemology - study of the nature of knowledge.
- Existentialism - philosophy of being/existence.
- "Biography of Ernst Heinrich Haeckel, 1834–1919" (article), Missouri Association for Creation, Inc., based on 1911 Britannica, webpage: Gennet-Haeckel: life, career & beliefs.
- "Colorado Symphony Orchestra - Richard Strauss (1864–1949): Also Sprach Zarathustra" (program notes), Charley Samson, Colorado Symphony Orchestra, 2004, webpage: CSO-AlsoSprach.
- "Classic Records Catalog / LSC-1806: Liner Notes" (description), Chicago Symphony Orchestra, R. D. Darrell, Radio Corporation of America (RCA), 1960, webpage: CSO-AlsoSprach.
- "KELVIN SMITH LIBRARY" (about Haeckel book on Monism), Case Western Reserve University, Cleveland, Ohio, 2004, webpage: CaseEdu-HaeMon00: notes Monism book as dated 1895.
- "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.
- Finkelstein, Gabriel Ward (2013). Emil du Bois-Reymond: Neuroscience, Self, and Society in Nineteenth-Century Germany. Cambridge, Massachusetts: The MIT Press. p. 272. ISBN 9780262019507.
- Ernst Haeckel, The Riddle of the Universe (Die Welträthsel or Die Weltraetsel, 1895–1899), Publisher: Prometheus Books, Buffalo, New York, 1992, reprint edition, paperback, 405 pages, illustrated, ISBN 0-87975-746-9.
- 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).