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.