World riddle
World riddle (Welträtsel in German) 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 (1895–1899), later published in English as The Riddle of the Universe (1901). 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.