The Nature of Time


Photograph by Ezer Pamintuan

Time is the inextricable foundation of our lives. We experience ourselves as if we were perpetually moving towards the future and away from a disappearing past, through a present moment that vanishes as soon as it arrives. Hence, we remember the past and anticipate the future, either with excitement or with dread. We go about our days, weeks, and months guided by cycles, routines, and projects that stretch out, are completed, and repeat themselves, and our lives as a whole are divided up into stages of childhood, adolescence, adulthood, and old age. The fact that time is so fundamental to how we experience ourselves and live our lives is so crucial that Martin Heidegger regarded time as the very “Being” of the human being. Yet given how deeply time pervades our lives, it is all the more baffling how mysterious it is. In his Confessions, St. Augustine writes, “What then is time? If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know.” In this post, I will explore three competing theories that attempt to unravel the mysterious nature of time. These are the A-theory, the B-theory, and the Moving Spotlight View.


Two Ways to Describe Time

Before I can dive into these three theories, I must make a few preliminary distinctions. First, I need to distinguish between two different ways that one can describe time and events. The first way is by describing time and events in terms of the notions of past, present, and future. J. M. E. McTaggart names this essentially tensed description of time the “A-series”. Examples of statements that use the A-series include “Barack Obama became president seven years ago”, “The Golden State Warriors and the Cleveland Cavaliers are competing in the NBA Finals”, and “Four years from now, the Summer Olympics will take place in Tokyo”.

The alternative way to describe time and events is to exclude any reference to the present, and any mention of the past and the future. The only features of events that are mentioned are their temporal relations to other events, and perhaps their ordering within a date system. McTaggart names this tenseless description the “B-series”. Unlike the A-series, which makes use of the notions of past, present, and future, the B-series uses the notions of “before”, “during” (or “simultaneous with”), and “after”. Examples of statements that use the B-series include “Over eight years after voting against the Iraq War, Barack Obama becomes president”, “The Golden State Warriors and the Cleveland Cavaliers meet in the NBA Finals in 2016”, and “The Tokyo Olympics takes place one year before Obama’s 60th birthday”. Notice that from these latter statements alone there is no way to know what the present time is, or which events are in the past or in the future.

(Here’s some hairsplitting: Although these statements are written in the present tense, this is a result of the English language’s lack of a tenseless inflection. I ask the reader to interpret the verbs in a way that does not necessarily peg them to the present moment. Instead, they speak generally about what objectively exists, in an absolute way – I call it the “objective tense”.)


What Aspects of Time Are Really Real?

Once these two descriptions of time and events have been distinguished, it becomes pertinent to ask which of them are necessary and sufficient to describe all of the objective features of reality. According to the Tensed View, the A-series describes objective features of time, and thus forms an indispensable part of a complete description of reality. It is objectively the case that 2016 is the current year, 2015 is in the past, and 2017 is in the future, and therefore A-series descriptions must supplement B-series descriptions in order to accommodate these facts. For those who hold the Tensed View, the B-series, by itself, leaves out a crucial fact about reality, namely, what time it is now.

On the other hand, the Tenseless View holds that the B-series by itself provides a complete, comprehensive description of reality. References to the past, present, and future are, on this view, superfluous and purely conventional, having no reference to how the world is in itself. The only facts about events that are objectively true are those involving relations of being “before”, “simultaneous with”, and “after” other events.


What Exists?

The final distinction that I will make is the distinction between Presentism and Eternalism, which give different accounts of what objects and events exist in objective reality.

According to Presentism, the only objects and events that truly and objectively exist are those that exist in the present moment. In other words, a complete and comprehensive inventory of the contents of reality would only list present objects and events. In contrast, Eternalism holds that all moments in time in the history of the universe exist in some way. So an Eternalist’s inventory of reality would include the residents of the Roman Empire, as well as the residents of the Mars Colonies (assuming such things are eventually built).


Three Theories of the Nature of Time

I now have the resources to describe the three theories of time. Each theory combines a view of what kinds of descriptions are necessary and sufficient for providing a complete description of reality (the Tensed View vs. the Tenseless View) with a view of what objects and events objectively exist (Presentism vs. Eternalism).

The A-theory combines the Tensed View with Presentism. For the A-theorist, the totality of reality wholly consists of present objects and events. The terms “present” and “existing” are co-extensive. Therefore, past and future objects and events strictly speaking do not exist at all. To be sure, past moments, and the objects and events that occupied them, used to exist, but they have since gone out of existence. Similarly, no future objects exist, although they will come into existence. (It is important to recall my earlier note: The A-theorist is not merely saying that these things don’t exist now, but that they don’t exist, period.) It’s not that the A-theorist can’t talk about past events such as the Battle of Gettysburg, or future events such as the inauguration of the 50th president of the United States. Indeed they can. But A-theorists must use A-series descriptions to talk about the past (or the future), allowing them to make explicit that the things they are talking about used to exist, but do not exist any longer (or will exist in the future).

The B-theory, on the other hand, combines the Tenseless View with Eternalism. For B-theorists, the dimension of time is analogous to the dimension of space. Just like every point in space is equally real, regardless of whether one is there to observe it or not, likewise every moment in time is equally real, even if we are not there to call it “the present”. If a transcendent being such as God were to look upon the universe, he would see the entire history of the universe, from the beginning of time to the end of time, and he would not identify any one particular moment as the present. For the B-theorist, “now” and “present” are indexicals like “here” – they are relative to the speaker uttering them. But just like New Yorkers are no less real simply because they are over 3,000 miles away from me, similarly, Ancient Egyptians are no less real simply because they are over 3,000 years “away” from me, in the direction of the past.

Finally, the Moving Spotlight View paves a middle way between the A-theory and the B-theory in that it combines the Tensed View with Eternalism. Similarly to the B-theory, the Moving Spotlight View holds that every moment of time exists in some manner. But like the A-theory, it insists that the present moment enjoys a privileged status. Present objects and events are “highlighted,” and when we say they are present, they are objectively so. The present moment moves across the timeline, so that moments that were once in the future eventually get their turn to become the present, and then instantaneously become part of the past. The name of this theory comes from the illustration of a firefighter’s spotlight moving from house to house. A better analogy is to picture a YouTube video with a timeline across the bottom, and a cursor that moves through the timeline from left to right. As the cursor moves, the part of the video that it is highlighting is played on the screen. For our purposes, the cursor can be thought of as the present moment, and the contents of the screen can be thought of as present objects and events.

Now let me ask you these questions: Which of these theories do you find intuitive or initially plausible? Which do you tacitly assume in the background of your life? Are any of them more “commonsensical” than the others? Can you think of any practical consequences, both positive and negative, of believing each of these theories? And finally, but most importantly, what rational arguments can you think of both for and against each of these theories?






_Davies, Paul. “That Mysterious Flow”. Scientific American. Sep. 2002: 40-47. Web. 29 Mar. 2014.

_Lee, Geoffrey. Metaphysics. Philosophy 125. Undergraduate lecture course. UC Berkeley. Spring 2014.

_McTaggart, J. M. E. The Nature of Existence. Cambridge, UK: Cambridge UP, 1927. pdf.

_Zimmerman, Dean. “The Privileged Present: Defending an ‘A-theory’ of Time”. Contemporary Debates in Metaphysics. ed. by Ted Sider, John Hawthorne, and Dean Zimmerman. Malden, MA: Blackwell, 2007. 211-225. pdf.





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