Four-Dimensionalism: An Ontology of Persistence and Time
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[1] In analytical metaphysics, there are three, closely related, debates about time and the nature of change and persistence. The first is about what there is. Presentists believe that only present things exist, whereas eternalists think that also past and future things exist, even though they do not exist now. The second debate is a debate about persistence: how is it that objects persist, and remain the same while undergoing change? Perdurantists say that this is because objects are genuinely four-dimensional, and are composed of temporal parts. Objects persist through time by perduring: the change of an object consists in the fact that it has temporal parts that have different properties. Endurantists deny this: objects are wholly present whenever they exist, and are therefore three-dimensional in nature. They persist through time by enduring: a changing object is wholly present at different times, and acquires some new property, which it then has in a special way, related to that particular moment in time. The third debate is the old discussion between an A-theory and a B-theory of time. A-theorists hold that change consists in events becoming Present after having been Future, and then becoming Past after having been Present: time is essentially tensed. B-theorists think that events are related by an ordering relation of ‘earlier than’, but that events have no intrinsic properties like ‘being Present’ or ‘being Past’. Therefore, any tensed statement can be reduced to some tenseless statement, or, at least, has tenseless truth-conditions. [2] Theodore Sider has written a forceful and lucid defense of the view that objects are four-dimensional in that they have temporal parts. The idea is best illustrated by noticing that change involves prima facie inconsistent propertyascriptions. An object may be red, but at some later time, it may be green. How can we ascribe both these properties to the same object? The answer is, of course, that the object has these properties at different times. So, one way of resolving the apparent inconsistency is by relativizing all properties to times. The four-dimensionalist however, chooses not to read the sentence ‘S is P at t’ as: ‘S is P-at-t’, but writes it as: ‘S-at-t is P’. Property ascriptions are timeless, but we ascribe them to objects-at-times, which are understood as temporal parts of four-dimensional objects. Objects can have different colors at once because their spatial parts are differently colored, but they can also have different colors