Perdurantism or perdurance theory is a philosophical theory of persistence and identity. The perdurantist view is that an individual has distinct temporal parts throughout its existence. (As opposed to endurantism, which is the view that an individual is wholly present at every moment of its existence). The use of "endure" and "perdure" to distinguish two ways in which an object can be thought to persist can be traced to David Kellogg Lewis (1986). However, contemporary debate has demonstrated the difficulties in defining perdurantism (and also endurantism). For instance, the work of Ted Sider (2001) has suggested that even enduring objects can have temporal parts, and it is more accurate to define perdurantism as being the claim that objects have a temporal part at every instant that they exist. Zimmerman (1996) has said that this won't work, as there have been many self-professed perdurantists who believe that time is 'gunky' and that for every interval of time, there is a sub-interval. Some perdurantists think this means there are no instants, since they define these as intervals of time with no subintervals. Currently there is no universally acknowledged definition of perdurantism (see also McKinnon (2002) and Merricks (1999)).