 In the eighteenth and nineteenth centuries, the status of teleology in biology was  also contested as part of the vitalist-mechanist debate in physiology  and medicine (for an overview, see the first section of the entry on  life).  Whereas mechanists sought to describe all living things in purely mechanical terms, vitalists argued that physical properties alone could not explain the goal-directed organization of living things. They claimed that ‘vital forces’ were also necessary to explain the difference between physical and living systems. Although they fell out of favor, some vitalist accounts persisted through the twentieth-century, for instance, in philosopher Henri Bergson’s ‘élan vital’ (see the section creative evolution in the entry on  Bergson)  and in biologist-cum-philosopher Hans Driesch’s concept of ‘entelechy’ (Driesch 1908). 