B-theory of time

The B-theory of time, also called the "tenseless theory of time", is one of two positions regarding the temporal ordering of events in the philosophy of time. B-theorists argue that the flow of time is only a subjective illusion of human consciousness, that the past, present, and future are equally real, and that time is tenseless: temporal becoming is not an objective feature of reality. Therefore, there is nothing privileged about the present, ontologically speaking.

The B-theory is derived from a distinction drawn by J. M. E. McTaggart between A series and B series. The B-theory is often drawn upon in theoretical physics, and is seen in theories such as eternalism.

The terms A-theory and B-theory, first coined by Richard M. Gale in 1966, derive from Cambridge philosopher J. M. E. McTaggart's analysis of time and change in "The Unreality of Time" (1908), in which events are ordered via a tensed A-series or a tenseless B-series. It is popularly assumed that the A-theory represents time like an A-series, while the B-theory represents time like a 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. Some philosophers have criticised hybrid theories, where one holds a tenseless view of time but asserts that the present has special properties, as falling foul of McTaggart's paradox. For a thorough discussion of McTaggart's paradox, see R. D. Ingthorsson (2016).

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 or flux. 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.^{[citation needed]}

The difference between A-theorists and B-theorists is often described as a dispute about temporal passage or 'becoming' and 'progressing'. B-theorists argue that this notion is purely psychological. 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. Another way to characterise the distinction revolves around what is known as the principle of temporal parity, the thesis that contrary to what appears to be the case, all times really exist in parity^{[definition needed]}. A-theory (and especially presentism) denies that all times exist in parity, while B-theory insists all times exist in parity.

B-theorists such as D. H. Mellor and J. J. C. Smart wish to eliminate all talk of past, present and future in favour of a tenseless ordering of events, believing the past, present, and future to be equally real, opposing the idea that they are irreducible foundations of temporality. B-theorists also argue that the past, present, and future feature very differently in deliberation and reflection. For example, we remember the past and anticipate the future, 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.

Earlier B-theorists argued that one could paraphrase tensed sentences (such as "the sun is now shining", uttered on September 28) into tenseless sentences (such as "on September 28, the sun shines") without loss of meaning. Later B-theorists argued that tenseless sentences could give the truth conditions of tensed sentences or their tokens. Quentin Smith argues that "now" cannot be reduced to descriptions of dates and times, because all date and time descriptions, and therefore truth conditionals, are relative to certain events. Tensed sentences, on the other hand, do not have such truth conditionals. The B-theorist could argue that "now" is reducible to a token-reflexive phrase such as "simultaneous with this utterance", yet Smith states that even such an argument fails to eliminate tense. One can think the statement "I am not uttering anything now", and such a statement would be true. The statement "I am not uttering anything simultaneous with this utterance" is self-contradictory, and cannot be true even when one thinks the statement. Finally, while tensed statements can express token-independent truth values, no token-reflexive statement can do so (by definition of the term "token-reflexive"). Smith claims that proponents of the B-theory argue that the inability to translate tensed sentences into tenseless sentences does not prove A-theory.