Four-dimensionalism

{{Short description|Metaphysical views about temporal parts and persistence over time}} {{About|the metaphysical doctrine|four-dimensional space|Four-dimensional space}}

In philosophy, '''four-dimensionalism''' (sometimes called the '''doctrine of temporal parts''') is a family of views about the ontology of time and persistence. Roughly, four-dimensionalists hold that persisting objects are extended in time in a way analogous to their extension in space, and that they are composed of distinct '''temporal parts''' located at different times, in addition to their spatial parts.<ref name="Sider1997">{{cite journal |last=Sider |first=Theodore |year=1997 |title=Four-Dimensionalism |journal=The Philosophical Review |volume=106 |issue=2 |pages=197–231 |doi=10.2307/2998357 |jstor=2998357}}</ref><ref name="HawleySEP">{{cite encyclopedia |last=Hawley |first=Katherine |title=Temporal Parts |encyclopedia=The Stanford Encyclopedia of Philosophy |editor=Edward N. Zalta |edition=Winter 2010 |publisher=Metaphysics Research Lab, Stanford University |year=2010 |url=https://plato.stanford.edu/archives/win2010/entries/temporal-parts/}}</ref><ref name="Costa2020">{{cite encyclopedia |last=Costa |first=Damiano |year=2020 |title=Persistence in Time |encyclopedia=Internet Encyclopedia of Philosophy |url=https://iep.utm.edu/per-time/}}</ref>

The label ''four-dimensionalism'' is used in more than one way in the literature. In a narrow sense it refers to theories of persistence—most prominently perdurantism and the closely related '''stage theory''' or ''exdurantism''—according to which ordinary objects persist by having temporal parts.<ref name="Sider2001">{{cite book |last=Sider |first=Theodore |year=2001 |title=Four-Dimensionalism: An Ontology of Persistence and Time |publisher=Oxford University Press |location=Oxford |isbn=0-19-926352-3}}</ref><ref name="Hawley2001">{{cite book |last=Hawley |first=Katherine |year=2001 |title=How Things Persist |publisher=Oxford University Press |location=Oxford |isbn=0-19-924913-X}}</ref> In a broader sense, the term is sometimes used for any view on which past, present and future times—and the objects located at them—are all equally real, in opposition to presentism. On this usage, "four-dimensionalism" functions as a label for non-presentist eternalist views of time rather than for a specific account of persistence.<ref name="Rea2003">{{cite book |last=Rea |first=Michael C. |year=2003 |chapter=Four-Dimensionalism |editor=Michael J. Loux |editor2=Dean W. Zimmerman |title=The Oxford Handbook of Metaphysics |publisher=Oxford University Press |pages=246–280}}</ref><ref name="Zimmerman2011">{{cite book |last=Zimmerman |first=Dean W. |year=2011 |chapter=Presentism and the Space-Time Manifold |editor=Craig Callender |title=The Oxford Handbook of Philosophy of Time |publisher=Oxford University Press |pages=163–244}}</ref>

Four-dimensionalist views play a central role in contemporary debates about identity over time, the nature of temporal reality and the interpretation of modern physics. They are typically contrasted with three-dimensionalism or endurantism, according to which persisting objects are wholly present at each time at which they exist and do not have temporal parts.<ref name="HawleySEP" /><ref name="Costa2020" />

== Terminology and definitions ==

Four-dimensionalists commonly appeal to the notion of a '''temporal part'''. A temporal part of an object is, roughly, a part that exists at some but not all of the times at which the object exists, and that stands to the whole as an ordinary spatial part stands to a spatially extended object.<ref name="HawleySEP" /><ref name="Heller1984">{{cite journal |last=Heller |first=Mark |year=1984 |title=Temporal Parts of Four Dimensional Objects |journal=Philosophical Studies |volume=46 |issue=3 |pages=323–334 |doi=10.1007/BF00372910}}</ref> Some writers take temporal parts to be instantaneous, while others allow them to occupy short temporal intervals.<ref name="McGrath2007">{{cite journal |last=McGrath |first=Matthew |year=2007 |title=Temporal Parts |journal=Philosophy Compass |volume=2 |issue=5 |pages=730–748 |doi=10.1111/j.1747-9991.2007.00094.x}}</ref>

The dispute between three- and four-dimensionalists is often framed in terms of how objects are located in spacetime. On one influential formulation, four-dimensionalism is the thesis that whenever an object exists across an extended stretch of time, there are distinct temporal parts of it corresponding to each sub-interval of that stretch; three-dimensionalism denies this and maintains that persisting objects are wholly present whenever they exist.<ref name="Sider1997" /><ref name="HawleySEP" />

Because the label "four-dimensionalism" is used in different ways, authors sometimes adopt terminological stipulations. Sider uses ''four-dimensionalism'' for the doctrine that objects persist by having temporal parts—that is, for perdurantism in his sense.<ref name="Sider1997" /><ref name="Sider2001" /> By contrast, Rea reserves the term "four-dimensionalism" for the denial of presentism, and uses ''perdurantism'' for the claim that persisting objects are not wholly present at every time at which they exist.<ref name="Rea2003" />

== Four-dimensionalism about material objects ==

On one common understanding, four-dimensionalism is primarily a thesis about the nature of material objects. According to this view, material objects do not merely extend across regions of space but also extend across regions of time: they have both spatial and temporal parts and occupy four-dimensional regions of spacetime.<ref name="Heller1990">{{cite book |last=Heller |first=Mark |year=1990 |title=The Ontology of Physical Objects: Four-dimensional Hunks of Matter |publisher=Cambridge University Press |location=Cambridge}}</ref><ref name="Sider2001" /><ref name="Hawley2001" /> Persisting objects are sometimes described metaphorically as "space–time worms" whose temporal dimension corresponds to their career over time.

In this material-object sense, four-dimensionalism is compatible with different views about which times and objects exist. A theorist may hold that material objects have temporal parts while remaining neutral on whether only present objects exist, or whether past and future objects exist as well. In practice, however, four-dimensional views of material objects are usually combined with non-presentist ontologies of time.<ref name="Costa2020" /><ref name="Gilmore2016">{{cite journal |last=Gilmore |first=Cody |author2=Costa |author3=Calosi |year=2016 |title=Relativity and Three Four-Dimensionalisms |journal=Philosophy Compass |volume=11 |issue=2 |pages=102–120 |doi=10.1111/phc3.12308|first3=Claudio|first2=Damiano|author-link3=Claudio Calosi}}</ref>

== Four-dimensionalism and theories of time ==

Four-dimensionalism is closely connected to debates in the philosophy of time about which times and events exist.

=== Eternalism, growing block, and presentism ===

Eternalists hold that past, present and future times, and the objects and events located at them, are all equally real. On this view, there is no ontologically privileged present, although there may still be objective relations of earlier-than and later-than.<ref name="MarkosianTime">{{cite encyclopedia |last=Markosian |first=Ned |title=Time |encyclopedia=The Stanford Encyclopedia of Philosophy |editor=Edward N. Zalta |edition=Winter 2020 |publisher=Metaphysics Research Lab, Stanford University |year=2020 |url=https://plato.stanford.edu/entries/time/}}</ref> Eternalism is often combined with four-dimensionalism about material objects, yielding a "block universe" picture in which reality consists of a four-dimensional spacetime filled with four-dimensional objects.<ref name="Sider2001" /><ref name="Costa2020" />

By contrast, presentists claim that only present objects and events exist, in the most fundamental sense; past and future things are said not to exist at all, or to exist only in a derivative or abstract way.<ref name="Presentism">{{cite encyclopedia |last=Ingram |first=David |title=Presentism |encyclopedia=The Stanford Encyclopedia of Philosophy |editor=Edward N. Zalta |edition=Fall 2018 |publisher=Metaphysics Research Lab, Stanford University |year=2018 |url=https://plato.stanford.edu/entries/presentism/}}</ref> On Rea's usage, four-dimensionalism is simply the denial of presentism, and thus includes both eternalism and views such as the growing block universe theory, on which the past and present exist but the future does not.<ref name="Rea2003" />

Although eternalism and four-dimensionalism are conceptually distinct, many arguments for four-dimensionalism appeal to features of relativistic spacetime that are also taken to support eternalism, such as the lack of an absolute, theory-independent notion of the present and the geometrical treatment of time in modern physics.<ref name="Zimmerman2011" /><ref name="Gilmore2016" />

== Four-dimensionalism and persistence ==

Four-dimensionalism, in the narrower, persistence-theoretic sense, concerns how objects persist through time and bear properties at different times.

=== Perdurantism (the "worm view") ===

Perdurantism (or '''perdurance theory''') claims that persisting objects are four-dimensional aggregates, or ''fusions'', of temporal parts.<ref name="Lewis1986">{{cite book |last=Lewis |first=David |year=1986 |title=On the Plurality of Worlds |publisher=Blackwell |location=Oxford}}</ref><ref name="Hawley2001" /><ref name="Costa2020" /> On this view, an ordinary object persists by having different temporal parts at different times, and facts about change are explained by the differing properties of those temporal parts rather than by a single wholly present thing having incompatible properties at different times. Because of this, perdurantism is sometimes called the "worm view": a persisting object is identified with the entire spacetime "worm" composed of its temporal parts.<ref name="HawleySEP" /><ref name="Heller1990" />

Perdurantism is usually taken to be a paradigmatic four-dimensionalist position about persistence. Many four-dimensionalists understand ''four-dimensionalism'' simply as perdurantism, together with the claim that all material objects persist in this manner.<ref name="Sider1997" /><ref name="Sider2001" />

=== Stage theory (exdurantism) ===

'''Stage theory''' or '''exdurantism''' is a closely related four-dimensionalist theory that also posits temporal parts, but differs in how it identifies ordinary objects.<ref name="SiderStage">{{cite journal |last=Sider |first=Theodore |year=1996 |title=All the World's a Stage |journal=Australasian Journal of Philosophy |volume=74 |issue=3 |pages=433–453 |doi=10.1080/00048409612347421}}</ref><ref name="Sider2000">{{cite journal |last=Sider |first=Theodore |year=2000 |title=The Stage View and Temporary Intrinsics |journal=Analysis |volume=60 |issue=1 |pages=84–88 |doi=10.1111/1467-8284.00144}}</ref> According to stage theory, the object ordinarily referred to at a time is an instantaneous (or short-lived) temporal stage, not the entire spacetime worm. Other stages at different times are related to the current stage by a suitable "temporal counterpart" relation, analogous to the counterpart relation used in some modal metaphysics.<ref name="SiderStage" /> Statements about an object's past and future are analysed in terms of these temporally related counterparts.

Stage theory thus offers a four-dimensionalist account of persistence and change, but denies that persisting objects are identical with extended spacetime worms. Many authors therefore treat perdurantism and exdurantism as distinct four-dimensionalist options.<ref name="Costa2020" /><ref name="McGrath2007" />

=== Endurantism and three-dimensionalism ===

'''Endurantism''' (or '''three-dimensionalism''') is the main rival to four-dimensionalist theories of persistence. Endurantists hold that ordinary objects are wholly present at each time at which they exist and do not have distinct temporal parts.<ref name="HawleySEP" /><ref name="Costa2020" /> A persisting object is numerically the same entity at each moment of its existence, even though it may have different properties at different times.

The dispute between endurantism and four-dimensionalist theories such as perdurantism and stage theory concerns how to understand the relation between objects, time and change. It also has implications for questions about material constitution (for example, statue–lump cases), identity, and the semantics of tensed discourse.<ref name="HawleySEP" /><ref name="McGrath2007" />

== A-series, B-series and four-dimensionalism ==

The contemporary debate about four-dimensionalism is often framed using J. M. E. McTaggart's distinction between the '''A-series''' and '''B-series''' of temporal positions.<ref name="McTaggart1908">{{cite journal |last=McTaggart |first=J. M. E. |year=1908 |title=The Unreality of Time |journal=Mind |volume=17 |issue=68 |pages=457–474 |doi=10.1093/mind/XVII.4.457}}</ref> The A-series orders events as future, present and past, and treats these tensed properties as changing with time. The B-series orders events only by the tenseless relations ''earlier than'', ''later than'' and ''simultaneous with'', which do not themselves change.<ref name="McTaggart1908" /><ref name="MarkosianTime" />

Four-dimensionalist views are typically associated with B-theoretic or tenseless approaches to time. Eternalists and other non-presentists often claim that the B-series structure of spacetime, especially as it appears in modern physics, supports a four-dimensionalist picture of reality in which past, present and future events all belong to a single four-dimensional block.<ref name="Zimmerman2011" /><ref name="Gilmore2016" /> By contrast, many A-theorists and presentists reject four-dimensionalism, arguing that an objective distinction between past, present and future is incompatible with the idea that objects are merely temporal worms or stages spread across time.<ref name="Presentism" />

== Arguments for four-dimensionalism ==

A number of influential arguments have been advanced in support of four-dimensionalist views.

=== Puzzles of change and material coincidence ===

One set of motivations arises from puzzles about change and material constitution. Classic examples include the Ship of Theseus and cases of coinciding objects such as a statue and the lump of clay from which it is made.<ref name="HawleySEP" /><ref name="Heller1984" /><ref name="Costa2020" /> If the statue and the lump share all their parts while the statue exists, they seem to occupy exactly the same region of spacetime yet differ in properties such as age and modal profile. Similar puzzles involve amoebas that divide, cats and their tail-complements, and other cases of apparent temporary or permanent coincidence.<ref name="Robinson1985">{{cite journal |last=Robinson |first=Denis |year=1985 |title=Can Amoebae Divide Without Multiplying? |journal=Australasian Journal of Philosophy |volume=63 |issue=3 |pages=299–319 |doi=10.1080/00048408512341901}}</ref>

Perdurantists and other four-dimensionalists claim that these puzzles are best resolved by treating persisting objects as four-dimensional entities with temporal parts. They can say, for example, that the statue and the lump partially overlap by sharing some temporal parts, or that one is a temporal part of the other, thereby explaining both our intuition that there is only one object in a given place at a time and the differences between the coinciding objects over their longer careers.<ref name="HawleySEP" /><ref name="Heller1990" /><ref name="McGrath2007" />

=== Temporary intrinsics and intrinsic change ===

Another line of argument appeals to the problem of '''temporary intrinsics'''—properties that an object appears to have independently of its relations to other things, but that can vary over time.<ref name="Lewis1986" /><ref name="Haslanger1989">{{cite journal |last=Haslanger |first=Sally |year=1989 |title=Endurance and Temporary Intrinsics |journal=Analysis |volume=49 |issue=3 |pages=119–125 |doi=10.1093/analys/49.3.119}}</ref> A familiar example is shape: a single object may seem to be first spherical and later cubical. On an endurantist view, this raises the question of how one and the same wholly present object can possess incompatible intrinsic properties.

David Lewis and other four-dimensionalists argue that the best response is to treat the apparently incompatible properties as belonging to different temporal parts of a four-dimensional object, much as spatially separated parts of an object can have different shapes.<ref name="Lewis1986" /><ref name="Lewis1976">{{cite book |last=Lewis |first=David |year=1976 |chapter=Survival and Identity |editor=Amélie O. Rorty |title=The Identities of Persons |publisher=University of California Press |location=Berkeley |pages=17–40}}</ref><ref name="Sider2000" /> On this view, the persisting object has temporal parts that are spherical and cubical, and there is no single thing that must be both at once.

=== Spacetime and relativity ===

Four-dimensionalist views are also motivated by considerations from special and general relativity, which treat space and time together as a four-dimensional spacetime manifold. The relativistic account of simultaneity makes it difficult to identify a unique, global present shared by all observers, and the geometry of spacetime is naturally represented using four-dimensional structures.<ref name="Gilmore2016" /><ref name="Zimmerman2011" /> Many authors therefore argue that the physics of spacetime supports some combination of (i) a four-dimensional spacetime ontology, (ii) a B-theoretic or tenseless view of time, and (iii) a four-dimensionalist account of persistence such as perdurantism.<ref name="Sider2001" /><ref name="Costa2020" />

Gilmore, Costa and Calosi (2016) distinguish several different "four-dimensional" theses suggested by relativity—about spacetime itself, about the ontology of time, and about persistence—and analyse how strongly the physics supports each.<ref name="Gilmore2016" />

== Criticisms and alternatives ==

Four-dimensionalism has been widely discussed and also widely criticised. Some critics argue that the notion of a temporal part is obscure or metaphysically extravagant, and that the puzzles used to motivate four-dimensionalism can be solved without positing such entities.<ref name="Rea1998">{{cite journal |last=Rea |first=Michael C. |year=1998 |title=Temporal Parts Unmotivated |journal=The Philosophical Review |volume=107 |issue=2 |pages=225–260 |doi=10.2307/2998484}}</ref><ref name="McGrath2007" /> Others contend that four-dimensionalism conflicts with common-sense intuitions about identity over time or with certain religious or ethical views about persons.

Endurantist accounts of persistence have been developed to meet many of the same puzzles that motivate four-dimensionalism, sometimes by appealing to constitution relations between coinciding objects, sometimes by revising principles about identity and parthood, and sometimes by adopting restricted ontologies that recognise fewer composite objects.<ref name="HawleySEP" /><ref name="Haslanger1989" /><ref name="Rea2003" /> Some presentists argue that a dynamic, tensed conception of time is more fundamental than any four-dimensionalist picture inspired by physics, and that presentism can be reconciled with modern spacetime theories.<ref name="Presentism" /><ref name="Zimmerman2011" />

Within four-dimensionalist camps, there are also disagreements. Stage theorists criticise the worm view for mislocating ordinary objects in spacetime, while worm theorists question whether stage theory can account adequately for our ordinary talk about persistence.<ref name="SiderStage" /><ref name="Costa2020" /> Some philosophers develop hybrid or alternative views—for example, "multi-location" or "transcendentist" theories—aimed at combining advantages of both three- and four-dimensional approaches while avoiding their respective problems.<ref name="Costa2020" /><ref name="Gilmore2016" />

== References ==

== Further reading ==

* {{cite book |last=Balashov |first=Yuri |year=2010 |title=Persistence and Spacetime |publisher=Oxford University Press |location=Oxford}} * {{cite book |last=Hudson |first=Hud |year=2001 |title=A Materialist Metaphysics of the Human Person |publisher=Oxford University Press |location=Oxford}} * {{cite book |last=Lewis |first=David |year=1986 |title=On the Plurality of Worlds |publisher=Blackwell |location=Oxford}} * {{cite book |last=Sider |first=Theodore |year=2001 |title=Four-Dimensionalism: An Ontology of Persistence and Time |publisher=Oxford University Press |location=Oxford}} * {{cite book |last=Hawley |first=Katherine |year=2001 |title=How Things Persist |publisher=Oxford University Press |location=Oxford}} * {{cite book |last=Loux |first=Michael J. |editor1-last=Loux |editor1-first=Michael J. |editor2-last=Zimmerman |editor2-first=Dean W. |year=2003 |title=The Oxford Handbook of Metaphysics |publisher=Oxford University Press |location=Oxford}} * {{cite book |last=Zimmerman |first=Dean W. |editor-last=Zimmerman |editor-first=Dean W. |year=2004 |title=Oxford Studies in Metaphysics, Volume 1 |publisher=Oxford University Press |location=Oxford}} * {{cite encyclopedia |last=Markosian |first=Ned |title=Time |encyclopedia=The Stanford Encyclopedia of Philosophy |editor=Edward N. Zalta |edition=Winter 2020 |publisher=Metaphysics Research Lab, Stanford University |year=2020 |url=https://plato.stanford.edu/entries/time/}} * {{cite encyclopedia |last=Hawley |first=Katherine |title=Temporal Parts |encyclopedia=The Stanford Encyclopedia of Philosophy |editor=Edward N. Zalta |edition=Winter 2010 |publisher=Metaphysics Research Lab, Stanford University |year=2010 |url=https://plato.stanford.edu/archives/win2010/entries/temporal-parts/}} * {{cite encyclopedia |last=Costa |first=Damiano |year=2020 |title=Persistence in Time |encyclopedia=Internet Encyclopedia of Philosophy |url=https://iep.utm.edu/per-time/}}

Category:Theories of time Category:Philosophy of physics Category:Spacetime