In his famous paper The Unreality of Time (1908), J. M. Ellis McTaggart argued that our perception of time is an illusion, and that time itself is merely ideal. He introduced the notions of the A-series and B-series interpretations of time, representing two different ways that events in time can be ordered.

The A-series corresponds to our everyday notions of past, present, and future. An A-series ordering involves statements such as X occurred in the past, X is occurring now, or X will occur in the future. This is contrasted with the B-series, in which events are placed in a chronological order according to relations of the form X occurred before Y, X occurred at the same time as Y, or X occurred after Y.

McTaggart argued that the A-series was a necessary component of any full theory of time, but that it was also self-contradictory and that our perception of time was therefore an ultimately incoherent illusion.