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Dynamic and Static Theories of Time

1.1. Dynamic versus Static Time

Time is familiar; it shapes our day-to-day lives; it provides direction and guidance to our planning; it underpins many of our attitudes to the world. And yet, despite being a fixture of everyday life, most of us couldn't offer much by way of a theory of what time is. Time is at once familiar and elusive. Our goal in this chapter is to provide an introduction to the modern debate about time within philosophy. We aim to equip the reader with the resources to begin to articulate, and even defend, a theory of time. We will also gesture towards some of the reasons why one might be tempted to endorse a particular theory of time; reasons that will be developed in greater detail in later chapters.

Our investigation into the philosophy of time begins with experience. Take a moment to reflect on your own experience of the world. Observe the shapes and colours around you; consider the sounds that float by on the wind; feel your own body and its ongoing process of exchange with the environment. Pause for a moment, and now search your own experiences for a sense of time. Is there anything in experience over and above the smells, sights and sounds that you can discern as distinctly temporal?

No? Well, try this. Meditate on your own mortality. Think about the swift hand of death that will one day come for you, as it comes for us all. Think, in particular, about the finitude of your own existence. One day, you will join every other organism that has existed on this planet in the dust. Now, think about all of the things that you like to do, all of the people you love and care about, and the desires you have as yet unfulfilled. There is not enough time in the world to do everything you want to do. You will die with goals unaccomplished, with words left unsaid, with love left unloved. As you ponder this, do you feel the moments begin to trickle by? Does time steal away your moments, bringing your death closer and closer?

It is this feeling, this sense that time is passing, pulling you inexorably towards your end, that constitutes one of the touchstones of the philosophy of time. Later, we will return to the experience of time passing and consider the extent to which it provides evidence for this or that theory of time. For now, we want you to try, as far as possible, to feel the flow of time.

By focusing on the experience of time, we can take our first steps towards developing an account of time. According to a dynamic approach to time, time really is as it seems to be in experience: the nature of time is fundamentally dynamic. Time really passes. It seems to us as though future events come ever closer until they become present, after which they recede further and further into the past. According to the dynamic theory of time, this seeming is to be taken at face value: time really does involve a kind of flow that reflects the way we experience the world.

The dynamic approach to time sits in stark contrast to a static approach to time. The static approach to time denies that time is fundamentally dynamic. Thus, while it may seem that future events slowly come towards us until they are present, thereafter receding into the past, this is not in fact how the world is. Time, on this conception, is fixed and never moves. Rather, it is we who, in some sense, move through time.

We will sharpen up the core distinction between the static and dynamic approaches to time in a moment. For now, we aim only to give the reader an intuitive sense of the distinction between the two views. So here's an analogy to help you develop your understanding. Consider the difference between a river and a road. A river has a current which sweeps anything caught in it downstream. A road, by contrast, has no current and does no sweeping. Rather, it is we who must make our way along the road, using whatever means of movement we might have available.

The dynamic approach to time treats time like a river. We all live our lives in the river of time, and it is the river that throws us to oblivion. The static approach to time treats time like a road. We must all wander the road, but the road itself does not move. Time, on this picture, doesn't bring death closer to us or bring us closer to death. Rather, it is we who bring ourselves closer to death by living.

1.2. Behind the Metaphor

Everything we have said so far has been couched in metaphor. While metaphors such as these are suggestive and can help to guide philosophy, ultimately one of the things that we, as philosophers, aim to do is get behind the metaphor. Doing so involves offering a theory of time. Before offering a theory of time, however, we need to take a step back and think carefully about what we are doing. What does it mean to offer a theory of time at all?

One possibility is that we are offering an account of what time is actually like. Another possibility is that we are offering an account of what is essential to time: an account of what it is for some phenomenon to be time. In order to understand this distinction we need to introduce some modal concepts. We need to distinguish between the way things are, and the way things could have been, but aren't. We can call the totality of ways things are, the actual world. Then we can call any complete way things could have been, but aren't, a merely possible world. So, for instance, there's a possible world in which you ate cornflakes for breakfast this morning, even though, actually, you ate toast. Which is to say that it's possible that you ate cornflakes, even though actually you ate toast.

We can now distinguish two rather different theories of time. We will call these constitutive theories and extensional theories. A constitutive theory of something aims to tell us what it is be that something. For example, a constitutive theory of dogs aims to tell us what it is to be a dog. Similarly, a constitutive theory of circles aims to tell us what it is to be a circle. So a constitutive theory of time aims to tell us what it is to be time. Translated into the language of possibility, what this means is that the constitutive theory of time aims to tell us what time is no matter how the world turns out to be. In short, we are trying to find the features that time has in every single possibility, where by 'possibility' we mean possibility in the widest sense: there is no possible way whatsoever for time to be different. This is sometimes called 'metaphysical' possibility, and is to be contrasted with 'physical possibility' which corresponds to what is possible given the physical laws of nature. By contrast, an extensional theory of something aims to tell us what that something is actually like. So an extensional theory of dogs aims to tell us whether there are any actual dogs, and if so, what they are like. An extensional theory of circles aims to tell us whether there are any actual circles, and if so, what they are like.

Constitutive theories of some phenomenon often aim to specify the necessary and sufficient conditions that must be met for something to be that phenomenon. A necessary condition for X's being Y is a condition that X must meet, if it is to be Y. A sufficient condition for X's being Y is a condition, which, if X meets it, then X is Y (but X can be Y without meeting that condition). So, for instance, you might think it is necessary for something to be a dog, that it has fur. Clearly, though, having fur is not sufficient for being a dog, since lots of things are furry, but are not dogs. Alternatively, you might think it is sufficient to be a dog that something has a particular sort of genetic profile, and is a member of a group with a particular evolutionary history, because if something has that genetic profile and is a member of a group with that evolutionary history, then it is a dog. Notice you might think this is sufficient, but not necessary to be a dog. Imagine a wizard waves his wand and creates something qualitatively just like a labradoodle. That thing didn't evolve; it's not a member of a group with a particular evolutionary history. But you might think it's a dog nonetheless.

It is easy to see why one might think that specifying both the necessary and sufficient conditions to be an X tells us what it is to be X. Once we've sorted out all the things that are necessary for being a dog, and all the things that are sufficient for being a dog, it seems like we know what it is to be a dog. Thought of this way then, constitutive theories of time are theories of time that aim to tell us the necessary and sufficient conditions for something to be time.

By contrast, an extensionalist theory of time aims to tell us what time is actually like. Consider again the case of dogs. An extensional theory of dogs might tell us a lot of things about dogs that the constitutive theory of dogs does not. It might tell us where dogs are actually found, what colours they are, how they behave, how they fit into society, what they eat, and so on. For instance, imagine that, actually, all dogs are either black or white. Then an extensional theory of dogs would tell us this, since it's an important feature of actual dogs that they only come in two colours. If you see something red out in the bush, running on four legs, you know it's not a dog. But that would be no part of a constitutive theory of dogs. A constitutive theory would not tell us that what it is to be a dog is to be either black or white. For...