to support the theory of egocentric presentism (or perspectival realism), which holds that there is a distinguished, present self. 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 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. Philosophical objections One main objection to presentism comes from the idea that what is true substantively depends upon what exists (or, that truth depends or 'supervenes' upon being). In particular, presentism is said to be in conflict with truth-maker theory, one theory which looks to capture the dependence of truth upon being with the idea that truths (e.g., true propositions) are true in virtue of the existence of some entity or entities ('truth-makers'). The conflict arises because most presentists accept that there are evidence-transcendent and objective truths about the past (and some accept that there are truths about the future, pace concerns about fatalism), but presentists deny the existence of the obvious truth-makers for such truths. For instance, most presentists accept that it