In metaphysics, A series and B series are two different descriptions of the temporal ordering relation among events. The two series differ principally in their use of tense to describe the temporal relation between events and the resulting ontological implications regarding time. John McTaggart introduced these terms in 1908, in an argument for the unreality of time. They are now commonly used by contemporary philosophers of time. History Metaphysical debate about temporal orderings reaches back to the ancient Greek philosophers Heraclitus and Parmenides. Parmenides thought that reality is timeless and unchanging. Heraclitus, in contrast, believed that the world is a process of ceaseless change, flux and decay. Reality for Heraclitus is dynamic and ephemeral. Indeed, the world is so fleeting, according to Heraclitus, that it is impossible to step twice into the same river. McTaggart's series McTaggart distinguished the ancient conceptions as a set of relations. According to McTaggart, there are two distinct modes in which all events can be ordered in time. A series In the first mode, events are ordered as future, present, and past. Futurity and pastness allow of degrees, while the present does not. When we speak of time in this way, we are speaking in terms of a series of positions which run from the remote past through the recent past to the present, and from the present through the near future all the way to the remote future. The essential characteristic of this descriptive modality is that one must think of the series of temporal positions as being in continual transformation, in the sense that an event is first part of the future, then part of the present, and then past. Moreover, the assertions made according to this modality correspond to the temporal perspective of the person who utters them. This is the A series of temporal events. Although originally McTaggart defined tenses as relational qualities, i.e. qualities that events possess by standing in a certain relations to something outside of time (that does not change its position in time), today it is popularly believed that he treated tenses as monadic properties. Later philosophers have independently inferred that McTaggart must have understood tense as monadic because English tenses are normally expressed by the non-relational singular predicates "is past", "is present" and "is future", as noted by R. D. Ingthorsson. B series From a second point of view, events can be ordered according to a