Four-dimensionalism

In philosophy, four-dimensionalism (also known as the doctrine of temporal parts) is the ontological position that an object's persistence through time is like its extension through space. Thus, an object that exists in time has temporal parts in the various subregions of the total region of time it occupies, just like an object that exists in a region of space has at least one part in every subregion of that space.

Four-dimensionalists typically argue for treating time as analogous to space, usually leading them to endorse the doctrine of eternalism. This is a philosophical approach to the ontological nature of time, according to which all points in time are equally "real", as opposed to the presentist idea that only the present is real. As some eternalists argue by analogy, just as all spatially distant objects and events are as real as those close to us, temporally distant objects and events are as real as those currently present to us.

Perdurantism—or perdurance theory—is a closely related philosophical theory of persistence and identity, according to which an individual has distinct temporal parts throughout its existence, and the persisting object is the sum or set of all of its temporal parts. This sum or set is colloquially referred to as a "space-time worm", which has earned the perdurantist view the moniker of "the worm view". While all perdurantists are plausibly considered four dimensionalists, at least one variety of four dimensionalism does not count as perdurantist in nature. This variety, known as exdurantism or the "stage view", is closely akin to the perdurantist position. They also countenance a view of persisting objects that have temporal parts that succeed one another through time. However, instead of identifying the persisting object as the entire set or sum of its temporal parts, the exdurantist argues that any object under discussion is a single stage (time-slice, temporal part, etc.), and that the other stages or parts that comprise the persisting object are related to that part by a "temporal counterpart" relation.

Though they have often been conflated, eternalism is a theory of what time is like and what times exist, while perdurantism is a theory about persisting objects and their identity conditions over time. Eternalism and perdurantism tend to be discussed together because many philosophers argue for a combination of eternalism and perdurantism. Sider (1997) uses the term four-dimensionalism to refer to perdurantism, but Michael Rea uses the term "four-dimensionalism" to mean the view that presentism is false as opposed to "perdurantism", the view that endurantism is false and persisting objects have temporal parts.