‘It was’—that is the name of the will’s gnashing of teeth and most secret melancholy.
Contemporary philosophers who study time disagree about its fundamental nature. Philosophical debate about this topic has in the last century been dominated by two competing views. These views are the A-theory and the B-theory of time.
According to the A-theory, times (e.g. the year 1908, the day you were born) objectively have tense properties like presentness, pastness, and futurity (these are called ‘A-properties’). For instance, 1908 objectively has the property of being in the past and the moment at which the sun dies objectively has the property of being in the future.
In contrast, B-theorists do not think that A-properties objectively apply to times. Rather, times only have A-properties relative to perspectives. For instance, 1908 is present relative to people in 1908, although of course it is not present relative to our perspective. The most we can objectively say, according to B-theory, is that some times are earlier than, later than, or simultaneous with others (these relations are called ‘B-relations’). For instance, 1908 objectively has the property of being earlier than 2019. This is not a perspectival fact; it was just as true for people living in 1908 as it is for us today.
We are creatures that are oriented towards the future and away from the past. Yet B-theorists argue that we should not project this idiosyncratic feature of human experience onto the objective world. The most powerful consideration against A-theory is that it looks to be inconsistent with physics. According to special relativity, there is no such thing as absolutely simultaneity, which means that for any two spatially separated events, there’s no perspective-independent answer as to whether those events occurred at the same time. It follows that there is no perspective-independent present and that A-theory is false. B-theory, however, is consistent with the relativity of simultaneity. For this reason, many (including me) believe that B-theory is the true theory of time. You probably should to.
As innocuous as this might sound, B-theory is actually quite weird. It forces us to rethink many of our beliefs about ourselves. For instance, it is natural to think that we are wholly present at each moment that we exist. But because they hold that there is no objective present moment for someone to be wholly located at, B-theorists are compelled to say that persons (and other ordinary objects) are stretched through time in much the same way that roads are stretched through space. You are a kind of spacetime worm that has both spatial parts and temporal parts (e.g. a part that is turning five, a part that is experiencing your first kiss). All your temporal parts are just as real from a tenseless, objective perspective as the one that is currently reading this blog. It’s just that they aren’t located here and now.
This idea is counterintuitive. Arthur Prior highlighted one aspect of its counterintuitiveness in an argument against B-theory. It goes something like this. Think of the most acutely painful experience you have ever had. When that experience ended, you probably felt a great deal of relief. It would have been reasonable for you to think, “Thank goodness that’s over!” Yet such a response is reasonable only if A-theory is true. For while it is reasonable to be relieved if the pain is objectively in the past, it’s not clear why you should be relieved by the mere fact that the pain occurred earlier than your thought, which is all it means to say that the pain is “over” according to B-theorists. Plus, if B-theory is correct, then your earlier temporal part is stuck with that pain, since it is tenselessly experiencing it. Rather than relief, it seems that you should feel horror and pity for your earlier temporal part.
Prior’s argument brings out in an especially forceful way how our thinking about our own lives occurs in an A-theoretic framework. Part of what it is to be a person with plans, projects, hopes, desires, regrets, reliefs, and so on is to think of oneself as moving towards the future and away from the past. For this reason, it is probably impossible to fully integrate B-theoretic thinking into one’s practical outlook.
Still, it seems to me that accepting B-theory should affect how one thinks about what it means to be a mortal with a finite amount of time left on this earth. B-theory entails that the passage of time is an illusion. Recognizing this casts in a softer, more equivocal light the feeling that one’s time is growing short, that one is, with each passing second, careening towards a moment wherein one will suddenly cease to be part of the furniture of the world. True, your current temporal part has considerably fewer temporal parts ahead of it than your five-year-old temporal part does. True, you will not exist at any times later than your death. But it is unclear why this should provoke any special regret or dread. For it’s not as if when you die, you suddenly become a fixture of the objective past while time and the present flow on without you. There is a very real sense in which your entire life—a short, vibrant streak across spacetime—constitutes an utterly indelible mark on the world. Your birth and death merely convey information about where in the universe you are located. It is sometimes intelligible to regret that you are in one place and not another (e.g. you might be sad you missed your cousin’s wedding). But none of this is worth getting too worked up about.
This outlook is not entirely stable for creatures like us, of course, and adopting it wholeheartedly, even if that were possible, would obscure a great deal of what is important in human life, like the bads of deprivation and decay and the goods of gain and progress. But I’ve found that in reflective moments I can adopt this mindset for a short time. And I’ve found that the results percolate into my less reflective moments. The thought of death and grief stings just a tiny bit less. And it seems to me that my choices somehow become imbued with a greater significance and permanence. For my actions and experiences do not pass away. They are not slowly enveloped by the fog of the past. They are, for those parts of me that live them, in a sense eternally recurrent. And this fact has the smell of something that matters.
 Thus Spoke Zarathustra, trans. Walter Kaufmann, Random House, p. 139 (1995).
 This distinction was introduced by J. M. E. McTaggart in “The Unreality of Time” Mind, vol. 17, no. 68, pp. 457-474 (1908).
 Not all A-theorists believe that the future and the past exist. For simplicity of explication I am going to restrict my discussion to a version of A-theory according to which the past and the future do exist (this version of A-theory is often called ‘the moving spotlight theory’).
 Strictly speaking, this view (called ‘four-dimmensionalism’) is not entailed by B-theory. But a B-theorist can deny it only by denying that ordinary objects persist through time.
 Prior, Arthur. “Thank Goodness That’s Over.” Philosophy, vol. 34, iss. 128, pp. 12-17 (1959)
 There is another side to Prior’s coin. If B-theory is correct, then every pleasurable experience you have ever had and will ever have is tenselessly being experienced by one of your temporal parts. Yet it might not make sense for you to look forward to those experiences, since the temporal part that is looking forward to those experiences will never experience them.