Presentism (philosophy of time)
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Presentism is the philosophical doctrine that only events and entities—and, in some versions of presentism, timeless objects or ideas like numbers and sets—that occur in the present exist. 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 Waterloo, and past entities, like Alexander the Great's warhorse Bucephalus, really do exist, though not in the present (eternalism, but not growing block theory, extends this to future events as well).
Saint Augustine proposed that the present is a knife edge between the past and the future and could not contain any extended period of time. This seems evident because, if the present is extended, it must have separate parts – but these must be simultaneous if they are truly part of the present. According to early philosophers, time cannot be both past and simultaneously present, so it is 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". Augustine proposed that God is outside of time and present for all times, in eternity. Other early philosophers who were presentists include the Buddhists (in the tradition of Indian Buddhism). A leading scholar from the modern era on Buddhist philosophy is Fyodor Shcherbatskoy, who 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' (also known as 'tensed time': yesterday, today, tomorrow) and the 'B Series' (or 'untensed time': Monday, Tuesday, Wednesday). Presentism entails 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 tensless facts (Dyke, 2004).
In the 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 events laid out in time as well as space. Different observers can disagree on whether two events at different locations occurred simultaneously depending if 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. However, 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 spacial relationships among objects; Thus observers need not be extended in time to exist and be aware, but rather they exist and the changes in internal relationships within the observer can be measured by stable countable events.
See also 
- Vol.1 of Buddhist Logic, 1962, Dover: New York. 70-71.
- Petkov 2005
- Balashov, Y., Janssen, M. (2002). Presentism and Relativity. Preprint of article in British Journal for the Philosophy of Science.
- Bourne, Craig (2006). A Future for presentism. Oxford: Oxford University Press.
- Four Dimensionalism. M.C.Rea. The Oxford Handbook for Metaphysics. Describes Presentism and how four dimensionalism contradicts it.
- McKinnon, N. (2003) Presentism and Consciousness. Australasian Journal of Philosophy. 81, 4. p305.
- Vesselin Petkov. (2005) Is There an Alternative to the Block Universe View?