In the 19th century, Haeckel developed a materialist form of hylozoism, specially against Rudolf Virchow's and Hermann von Helmholtz's mechanical views of humans and nature. In his "Die Welträtsel" of 1899 ("The Riddle of the Universe" 1901), Haeckel upheld a unity of organic and inorganic nature and derived all actions of both types of matter from natural causes and laws. Thus, his form of hylozoism reverses the usual course by maintaining that living and nonliving things are essentially the same, and by erasing the distinction between the two and stipulating that they behave by a single set of laws.