World riddle
Image:Ernst Haeckel 2.jpg|thumb|170px|right| Ernst Haeckel wrote about the World Riddle in 1895.
World riddle is a philosophical term concerning fundamental questions about the nature of the universe and the meaning of life. The term gained prominence in the late 19th century and is most closely associated with two key figures: the philosopher Friedrich Nietzsche and the biologist Ernst Haeckel.
Nietzsche mentioned Welträthsel in several of his writings, exploring profound existential questions. However, it was Haeckel who popularized the term with his influential book, Die Welträthsel, later published in English as The Riddle of the Universe. In this work, Haeckel attempted to resolve these riddles using a scientific and monistic worldview.
The World Riddle has also been explored as an inspiration or allegorical theme in some musical compositions, notably the unresolved harmonic progression at the end of Richard Strauss's 1896 tone poem, Also sprach Zarathustra.
View of Nietzsche
Image:FWNietzscheSiebe.jpg|thumb|120px|right| Friedrich NietzscheFriedrich Nietzsche referred to the "World Riddle" in several of his writings.
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 a lecture to the Berlin Academy of Sciences he declared that neither science nor philosophy could ever explain these riddles.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, in a lecture in 1892, would have a single answer since humans and the universe were contained within one system, a mono-system: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.