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In the philosophy of time,
presentism is the theory that only present things exist, and future and past things are unreal. Past and future "entities" are to be
construed as logical
constructions or fictions. The opposite of presentism is
'eternalism', which is
the belief that things in the past and things yet to come exist eternally.
Presentism is compatible with Galilean relativity, in which time is
independent of space but is probably incompatible with Lorentzian/Einsteinian relativity in
conjunction with certain other philosophical theses which many find
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
Stcherbatsky, 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 B Series facts can be explained or reduced to
A series facts. Presentists maintain that temporal discourse
requires the use of tenses.
In the modern theory of relativity, the
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 and has given 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.