Conventionally, time is divided into three distinct regions; the "past", the "present", and the "future". Using that representational model, the past is generally seen as being immutably fixed, and the future as undefined or nebulous at best. As time passes, the moment that was once the present becomes part of the past; and part of the future, in turn, becomes the new present. In this way time is said to pass, with a distinct present moment "moving" forward into the future and leaving the past behind.
Within this intuitive understanding of time is the philosophy of presentism, which argues that only the present exists. It does not travel forward through an environment of time, moving from a real point in the past and toward a real point in the future. Instead, the present simply changes. The past and future do not exist and are only concepts used to describe the real, isolated, and changing present.
This conventional model presents a number of difficult philosophical problems, and seems difficult to reconcile with currently accepted scientific theories such as the theory of relativity.
Since the 1990s there has been much debate in the philosophy of time between proponents of presentism and eternalism. Presentism is a version of the A-theory because it holds that there is a real ontological distinction between past, present and future, and that time’s flow is real. The presentist version of the ontological distinction between past, present and future is the most extreme: only what is present exists; neither past nor future entities exist. For example, according to presentism, Queen Elizabeth II exists, but Queen Elizabeth I does not, and neither does Queen Elizabeth II’s first great-great-grandchild.
Eternalism is the opposing view that past and future objects are just as real as present objects. Queen Elizabeth I and Queen Elizabeth II’s first great-great-grandchild are just as real as Queen Elizabeth II herself. Eternalism is, thus, the ontological component of the B-theory. However, it is possible to be an eternalist and an A-theorist, for example, by holding that past (future) objects exist but count as past (future) in virtue of possessing the property of pastness (futurity).
The focus on the presentism / eternalism debate is at least partly explained by the rise in popularity of presentism itself. Perhaps one explanation for this is due to an argument that undermines non-presentist versions of the A-theory. According to this argument, any theory that assigns ontological privilege to the present moment while also recognizing the existence of non-present times faces an insurmountable problem: it is unable to account for our knowledge that we are located in the present. If anything is certain, surely our knowledge that we are present is! But if past times exist as well as the present time, what is to say we are not located in one of those past times, mistakenly believing ourselves to be present? We might insist that our experience of presentness is so compelling that it must be veridical. But what about Queen Elizabeth I’s experience of presentness? That’s pretty compelling too, yet she is in the past, so her experience misleads her. Perhaps our experience misleads us too.
There are two views of the ontological nature of time that are immune to this argument. They are presentism and the B-theory’s version of eternalism. Presentism avoids the thrust of the argument by denying that there are any nonpresent times that are real. Since only what is present exists, if I exist, I am present. My knowledge that I am present is therefore guaranteed. The B-theory avoids it by denying that the present is ontologically privileged. My knowledge that I am present is nothing more than the indexical knowledge that I am located at the time at which I am located. Knowing that I am present is no more ontologically significant than knowing that I am here. I cannot mistakenly hold either of these beliefs.
Arguments against Eternalism (B-theories)
In order to bolster the view that the A-theory constitutes the common-sense, or default theory of time, the A-theorist often appeals to the fact that ordinary temporal language recognizes a distinction between past, present and future, and the associated passage of time. It is then argued that we should, barring strong defeating reasons, accept that ordinary language is veridical in this respect. This kind of argument can be seen as a challenge to the B-theorist to come up with strong defeating reasons why we should not accept that tensed language and thought is veridical, but it has also been offered explicitly as a positive argument in favour of the A-theory.
Early B-theorists argued that tensed language, that is, language that appears to locate events somewhere in the A-series, could be eliminated in favour of tenseless language that, instead, locates events in the B-series. So, for example, a tensed sentence such as ‘The first moon landing is past’, uttered in 2009 could be translated into a tenseless sentence such as ‘The first moon landing occurs before 2009’, where the ‘occurs’ is taken as tenseless rather than present tense. This reductionist strategy was intended to establish that, even though ordinary language is tensed, this should not be taken as indicating that temporal reality itself is tensed. Since tensed language can be eliminated in favour of tenseless language, it is not needed for a complete description of reality, so there is no feature of reality that it, alone, picks out. However, this reductionist strategy ultimately failed, as it was found that it is not the case that every tensed sentence can be translated without loss of meaning by a tenseless sentence. A-theorists took this to show that, since there is some aspect of the meanings of tensed sentences that cannot be captured by any tenseless sentence, there must also be some feature of reality that can only be described using tensed language, namely, facts about the pastness, presentness and futurity of events.
B-theorists, however, argued that, even if tensed language is irreducible, that is a fact about tensed language, not a fact about temporal reality. The real significance of tensed sentences is that some of them are true, so the important question is what is it about the world that makes them true? A-theorists think that tensed facts make tensed sentences true. B-theorists argue that tenseless facts can do the job. This change in strategy on the part of the B-theorists was initially proposed by Smart and Mellor in terms of the notion of truth conditions. They argued that the truth conditions of tensed sentences could be stated in a tenseless metalanguage. The point of this was to show that the conditions that must be met for a tensed sentence to be true can be met by purely tenseless facts. So, even though tense is an irreducible feature of language, the truth of true tensed sentences can be fully accounted for without the need to invoke the existence of tensed facts. Tenseless facts are sufficient to ground the truth of true, irreducibly tensed sentences. For instance, the sentence ‘The first moon landing is past’, uttered in 2009 is true if, and only if, the first moon landing occurs before 2009. So, the fact that the event referred to by a sentence stands in a certain tenseless temporal relation to the time at which the sentence is uttered is a necessary and sufficient condition for the truth of that sentence. More recent statements of this project have been developed in terms of truth makers.
It should be noted that neither the translation strategy of the old B-theorists, nor the truth-condition strategy of the new B-theorists for dealing with tensed language, should be seen as a positive argument in favour of the B-theory. No argument whose premises solely concern the nature of temporal language and whose conclusion concerns the nature of temporal reality can be valid. Instead, the aim of the B-theorists’ strategy is to show that the existence of irreducible tense in language need not be indicative of the existence of tense in reality. In other words, the B-theorist aims to undermine the A-theorist’s inference from the existence of tense in language to the existence of tense in reality.
As well as appealing to the fact that language and thought is tensed (and irreducibly so) to support their theory, A-theorists also appeal to the fact that our temporal experience strongly suggests that time is dynamic. This too can be seen as a challenge to the B-theorist to provide an adequate explanation of why we seem to experience time as flowing if it doesn’t really flow.
There are two features of our temporal experience that need to be explained if the B-theory is true. First, there is the fact that experience seems to occur only in the present. Second, there is the fact that we seem to experience time as flowing. B-theorists generally explain the first of these features by appeal to the fact that our span of direct awareness, or ‘specious present’, is very brief. Since, at any one time, we are only aware of what is going on in a very limited period of time, it can seem to us that what is located within that very brief time span is all that is really going on. However, just because we are unable to perceive events that are happening at other times does not mean they are not happening at those times.
The B-theorists’ explanation for the apparent experience of temporal passage is often given in terms of the fact that our memories accumulate over time. Suppose you perceive the hands of a clock showing first 2:00, then 2:15 and then 2:30. These experiences are not totally isolated from each other. Your perception of the clock showing 2:15 is accompanied by a memory of having already perceived it showing 2:00, and your perception of it showing 2:30 is accompanied by a memory of it showing 2:15 while remembering it showing 2:00. According to the B-theorist, our sense of the passage of time is, at least in part, to be explained by the fact that our memories of our experiences accumulate in this way.
Arguments against Presentism (A-theories)
In The Unreality of Time, J. M. E. McTaggart divided time into an A-series and a B-series, with the A-series describing events in absolute tensed terms (past, present, and future) and the B-series describing events in terms of untensed temporal relations (before and after). He also added the notion of a "C-series", a series that has an order but with no notion of time, like a series of letters. He went on to argue that the A-series was needed for anything deserving the name "time", since he argued that only the A-series can allow for genuine change, and he considered change to be an essential part of any reasonable definition of time. But, he argued, the A-series was logically incoherent, so he concluded that time was unreal, and since he also believed the B-series depended on the A-series, he also concluded that only the C-series could remain as a meaningful ordering.
McTaggart’s argument (often called McTaggart’s paradox) that the notion of the A-series involves a contradiction goes as follows. The first step is to note that the A-characteristics of pastness, presentness and futurity are incompatible with each other. Nothing can exemplify more than one of these characteristics. The next step is to consider the effect on that fact of the other aspect of an A-theoretic conception of time, namely, the fact that time is dynamic, so that what is future becomes present and then becomes past. When the flow of time is added to the picture, we find that, since events are continually changing their A-series locations, each one exemplifies every A-characteristic. But this is in direct conflict with the first step, that nothing can exemplify more than one A-characteristic.
The natural response at this point is to note that nothing exemplifies more than one A-characteristic at the same time, so there is really no contradiction between these two aspects of an A-theoretic conception of time. However, McTaggart anticipates this move. He argues that in order fully to cash out this response, we must specify when an event exemplifies each of the incompatible A-characteristics, and furthermore, we must do so in A-series terms. If, instead, we used B-series terms to specify when an event exemplified each of the incompatible A-series characteristics, our attempt to characterize the A-series would collapse into a characterization of the B-series. If, for example, we said that an event, e, is future on Sunday, present on Monday, and past on Tuesday, this description would be a permanent, unchanging one, equivalent to the B- theoretic description that e occurs on Monday, is later than Sunday and earlier than Tuesday. But this describes unchanging, B-series facts. So, instead, we would have to say that e is now present, will be past, and has been future. But this move, McTaggart argues, introduces compound tenses, and when we consider the entire range of compound tenses (there are nine of them), we can see that they are just as incompatible with each other as the three simple tenses. Furthermore, given the flow of time, every event exemplifies every compound tense. So, the contradiction has not been avoided by switching our attention from the three simple tenses to the nine compound tenses. Again, the reply could be made that no event exemplifies any of the incompatible compound tenses at the same time. But again, McTaggart can reply that specifying in A-series terms exactly when an event exemplifies the incompatible compound tenses generates a third level of even more complex tenses, and a similar incompatibility exists among them. Given the flow of time, every event exemplifies every third-level compound tense, so once more the contradiction has not been avoided. The regress that we have thus embarked upon is vicious. At no level is the contradiction removed.
The other main objection to the A-theory is that it is prima facie inconsistent with the special theory of relativity. Given that the special theory is a well-confirmed scientific theory, and thus is very likely to be true, it is incumbent upon any proponent of an A-theory of time to show that the two theories can, in principle, be reconciled. According to the standard interpretation of the special theory, simultaneity does not obtain absolutely, but only relative to a frame of reference.
Furthermore, no one frame of reference is privileged. It follows that there is no frame-independent way of marking out all those events that are simultaneous with the present moment, and thus are themselves present. An event which is present relative to one frame of reference may be past or future relative to another frame of reference, and neither of these frames of reference has a claim to being ontologically privileged. Consequently, there is no absolute (frame-independent) distinction between past, present and future. Furthermore, if presentness confers ontological status on events, then existence itself becomes a frame-relative matter. An event might exist, because it is present, relative to one frame of reference, yet not exist, because it is past or future, relative to another.
Some A-theorists like Saunders have attempted to reconcile their view with the special theory. Whether such attempts are successful is the subject of much contemporary debate.
"According to presentism, all that is physically real is the present—a system of physical events all of which are simultaneous with each other. No other events are real. Precisely what this system of events may be, now, as I snap my fingers, may not be known to me; but there is a fact of the matter as to what it is, and it is a universal fact which embraces us all. It is an intersubjective reality—now, as a snap my fingers—and it is a reality which contains us only as an incidental part. But even if one knew all that there is to know, consistent with special relativity, one would not be able to say what this system of events might be. According to presentism, therefore, special relativity is radically deficient as a description of reality. It is blind to the sequencing of what is physically real." (Saunders 2002: 279–80)
References
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Physics