B-theory of time
The B-theory of time is a term, given to one of two positions taken by theorists, in the philosophy of time. The labels, A-theory and B-theory, are derived from the analysis of time and change developed by Cambridge philosopher J. M. E. McTaggart in The Unreality of Time, in which events are ordered via a tensed A-series or a tenseless B-series.
Events (or 'times'), McTaggart observed, may be characterized in two distinct, but related, ways. On the one hand they can be characterized as past, present or future, normally indicated in natural languages such as English by the verbal inflection of tenses or auxiliary adverbial modifiers. Alternatively events may be described as earlier than, simultaneous with, or later than others. Philosophers are divided as to whether the tensed or tenseless mode of expressing temporal fact is fundamental. Those who (like Arthur Prior) take the tensed notions associated with the past, present and future to be the irreducible foundations of temporality and our conceptions of temporal fact, are called A-theorists (similar to presentists). A-theorists deny that past, present and future are equally real, and maintain that the future is not fixed and determinate like the past. Those who wish to eliminate all talk of past, present and future in favour of a tenseless ordering of events are called B-theorists. B-theorists (such as D.H. Mellor and J.J.C. Smart) believe that the past, the present, and the future are equally real.
The past, the present and the future feature vary differently in deliberation and reflection. We remember the past and anticipate the future, for example, but not vice versa. B-theorists maintain that the fact that we know much less about the future simply reflects an epistemological difference between the future and the past: the future is no less real than the past; we just know less about it (Mellor 1998). A view was held, for example by Quine and Putnam, that physical theories such as special relativity and latterly Quantum mechanics provide the B-theory with compelling support.  
A-theorists on the other hand believe that a satisfactory account of time must acknowledge a fundamental metaphysical difference between past, present and future (Prior 2003). The difference between A-theorists and B-theorists is often described as a dispute about temporal passage or 'becoming'. B-theorists argue that this notion embodies serious confusion about time, while many A-theorists argue that in rejecting temporal 'becoming', B-theorists reject time's most vital and distinctive characteristic. It is common (though not universal) to identify A-theorists' views with belief in temporal passage.
It is also common (though not universal) for B-theorists to be four-dimensionalists, that is, to believe that objects are extended in time as well as in space and therefore have temporal as well as spatial parts. This is sometimes called a time-slice ontology (Clark, 1978).
The debate between A-theorists and B-theorists is a continuation of a metaphysical dispute reaching 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. The metaphysical issues that continue to divide A-theorists and B-theorists concern the reality of the past, the reality of the future, and the ontological status of the present.
- Clark, M. (1978) Time-slices of particular continuants as basic individuals: An impossible ontology. Philosophical Studies 33, 403--408.
- Davies, Paul (1980) Other Worlds. Harmondsworth: Penguin.
- McTaggart, J.M.E. (1908) The Unreality of Time, Mind 17: 457-73.
- McTaggart, J.M.E. (1927) The Nature of Existence, Vol II. Cambridge: Cambridge University Press.
- Mellor, D.H. (1998) Real Time II. London: Routledge.
- Prior, A.N. (2003) Papers on Time and Tense. New Edition by Per Hasle, Peter Øhrstrøm, Torben Braüner & Jack Copeland. Oxford: Clarendon.
- Putnam, H. (2005) A Philosopher Looks at Quantum Mechanics Again, British Journal for the Philosophy of Science, 56, pp 615 - 634.
- Quine, W. V. O. (1960) Word and Object, Cambridge, MA: M.I.T. Press.
- Markosian, Ned, 2002, "Time", Stanford Encyclopedia of Philosophy
- Arthur Prior, Stanford Encyclopedia of Philosophy