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Philosophical presentism is the view that neither the future nor the past exist. In some versions of presentism, this view is extended to timeless objects or ideas (such as numbers). According to presentism, events and entities that are wholly past or wholly future do not exist at all. Presentism contrasts with eternalism and the growing block theory of time which hold that past events, like the Battle of Manzikert, and past entities, like Alexander the Great's warhorse Bucephalus, really do exist, although not in the present. Eternalism extends to future events as well.
Augustine of Hippo proposed that the present is analogous to a knife edge placed exactly between the perceived past and the imaginary future and does not include the concept of time. Proponents claim this should be self-evident because, if the present is extended, it must have separate parts – but these must be simultaneous if they are truly a part of the present. According to early philosophers, time cannot be simultaneously past and present and hence not extended. Contrary to Saint Augustine, some philosophers propose that conscious experience is extended in time. For instance, William James said that time is "the short duration of which we are immediately and incessantly sensible". Other early presentist philosophers include the Indian Buddhist tradition. Fyodor Shcherbatskoy, a leading scholar of the modern era on Buddhist philosophy, has written extensively on Buddhist presentism: "Everything past is unreal, everything future is unreal, everything imagined, absent, mental... is unreal. Ultimately, real is only the present moment of physical efficiency [i.e., causation]."
According to J. M. E. McTaggart's "The Unreality of Time", there are two ways of referring to events: the 'A Series' (or 'tensed time': yesterday, today, tomorrow) and the 'B Series' (or 'untensed time': Monday, Tuesday, Wednesday). Presentism posits that the A Series is fundamental and that the B Series alone is not sufficient. Presentists maintain that temporal discourse requires the use of tenses, whereas the "Old B-Theorists" argued that tensed language could be reduced to tenseless facts (Dyke, 2004).
Arthur N. Prior has argued against un-tensed theories with the following ideas: the meaning of statements such as "Thank goodness that's over" is much easier to see in a tensed theory with a distinguished, present now. Similar arguments can be made to support the theory of egocentric presentism (or perspectival realism), which holds that there is a distinguished, present self.
In modern theory of relativity, the conceptual observer is at a geometric point in both space and time at the apex of the 'light cone' which observes the events laid out in time as well as space. Different observers may disagree on whether two events at different locations occurred simultaneously depending on whether the observers are in relative motion (see relativity of simultaneity). This theory depends upon the idea of time as an extended thing and has been confirmed by experiment, thus giving rise to a philosophical viewpoint known as four dimensionalism. Although the contents of an observation are time-extended, the conceptual observer, being a geometric point at the origin of the light cone, is not extended in time or space. This analysis contains a paradox in which the conceptual observer contains nothing, even though any real observer would need to be the extended contents of an observation to exist. This paradox is partially resolved in relativity theory by defining a 'frame of reference' to encompass the measuring instruments used by an observer. This reduces the time separation between instruments to a set of constant intervals.
Some of the difficulties and paradoxes of presentism can be resolved by changing the normal view of time as a container or thing unto itself and seeing time as a measure of changing spatial relationships among objects. Thus, observers need not be extended in time to exist and to be aware, but they rather exist and the changes in internal relationships within the observer can be measured by stable countable events.