In philosophy, presentism is the belief that neither the future nor the past exists. The opposite of presentism is 'eternalism ', which is a belief that things that are past and things that are yet to come exist. Presentism is compatible with Galilean relativity, in which time is independent of space but is probably incompatible with Lorentzian/Einsteinian relativity. Presentism can also be used more loosely to refer to a narrow focus on the conditions of the moment.
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. If James were correct, then by this reasoning, God would be in us all.
According to J. M. E. McTaggart there are two ways of referring to events: the 'A Series' and the 'B Series' (see link). Later philosophers regard presentism as a belief that only the 'A Series' exist, and Presentists usually maintain that it only makes sense to refer to events with statements that have a definite tense.
In the modern theory of relativity, the conceptual observer is 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. The observer determines that different events are simultaneous depending upon the motion of the observation point. This theory depends upon the idea of time as an extended thing and has been confirmed by experiment. 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.
See also
Further reading