[Caveat: there is a large literature on the philosophy of time that I have not investigated carefully. The following thoughts may be naive.]
- Our habitat exhibits temporal regularities
Like all species on this planet, we evolved in an environment strongly marked by the regular movements of the earth and moon in the presence of the sun. Most species respond to these regularities–resting by night or by day, budding every spring, foraging at low tide. For species that are sentient, it pays to be aware of at least the most relevant regularities.
- We perceive rhythms
Human beings can sense temporal regularities. We can notice that a frog is croaking at an even pace or that each day is about as long as the next one. We can also make rhythmic sounds of our own. My colleague Anirudh Patel shows that some birds experience rhythm much as we do, but that monkeys do not. As hard as they try, monkeys cannot predict the next beat. It seems likely that our ways of perceiving and creating temporal regularities are adaptive for us, as highly social creatures living on a planet with pervasive temporal rhythms. For one thing, music and dance strengthen social bonds. But different species have adapted to the same context differently.
- Our ideas of time are deeply cultural
People are taught how to perceive time from a very early age. We learn words for minutes, days, and years. Our languages mark time in complex (but diverse) ways, such as verb tenses or adverbial expressions. We learn metaphors for time, such as the clock’s round face or a calendar’s rows and columns. And elements of time are deeply imbued with significance, sometimes even sacredness.
This inheritance makes it hard to tell whether individuals could perceive (or invent) temporal regularities all by themselves. When one of my daughters was about two, she went to nursery school every other weekday. She said that this pattern was a “stripe.” I don’t know whether she was reinventing the metaphor of time as a line divided into periods (in which case this metaphor could be hard-wired) or whether she was simply applying a metaphor that she had already learned from us. But at least some of our notions of time are cultural inheritances.
- Our direct perceptions of temporal regularities are imperfect
We can perceive that a clock’s ticks are spaced evenly, but if the clock slows down very gradually, we will not notice. We can perceive that the seasons rotate through an annual pattern that takes hundreds of days to complete, but I do not think we would notice that a year took 350 or 400 days if we didn’t measure and record the passage of time.
- We use tools to measure time
Because we can recognize temporal patterns, we can identify objects that are particularly regular and use them as measures by comparing them to other objects. For our ancestors, the regular motions of heavenly bodies provided reliable measures, and they erected objects that cast shadows to track these motions. Sometimes we simply perceive that something (such as water dripping through a narrow hole) is regular and use it as a measurement tool. But it is better to have an explanation of the object’s regularity. For instance, Newtonian physics can explain why the earth has rotated at an even pace during our recorded history. With modern physics, we can now invent quantum clocks. All of the objects that exhibit temporal regularities are consistent with each other.
- These five background conditions encourage us to model time in certain ways
We pervasively think of time as a line (whether straight, circular or oval), with the present as a point that moves along steadily. We may think that only the present point is real or actual. The past has left residues in the present, including effects on our nervous system that we can summon as memories. And the regularities of nature allow us to predict the future. But all of our direct perceptions are of the present, which has no duration and turns instantly into the past.
Such models may be contingent on our background conditions: our physical capacities, which evolved for a particular environment, plus the tools and cultural apparatus that we have developed. A different culture–let alone a different species on a different planet–might grasp time in fundamentally different ways that would work just as well.
On the other hand, I would not quickly assume that a linear model of time is “Western” or “modern,” whatever those categories mean. Writing ca. 400s CE, the Buddhist monk Buddhaghosa introduces a simile of a chariot wheel that touches the ground at one point while it spins, saying that the “life-moment of living beings” is equally brief. Although we perceive a moving wheel as a continuous thing, Buddhaghosa wants us to see each instant as discrete.
- Our sense of time causes distress
To varying degrees, we are troubled by feelings that time is running out, that we are wasting the present, that we would change the past if we could, that the future will be bad in certain ways, or that we wish we could directly experience what we did in the past (nostalgia).
- A linear model of time might be the root of the distress
These feelings seem rooted in the class of models that I mentioned above, which represent time as linear with the present as a point.
- Linear models do not describe our consciousness of time
A linear model of time works well to describe history or physical processes. It enables useful objects such as chronologies, calendars, and time-stamps. Because of its utility, we will always return to it. But it does not describe how we experience time.
As many have noted, if we only experienced the present, we could not hear a melody. We would only hear the current note or chord. We could not understand a sentence; we would only hear the word being uttered. And we could not catch (or duck) an incoming ball, because it would appear as a circle in our visual field. Evidently, we perceive objects that extend in time and change.
Kant thought that our perceptions were momentary, but there must be a persistent self that puts our perceptions together. William James disagreed that the perceptions themselves lack duration. “The practically cognized present is no knife-edge, but a saddle-back, with a certain breadth of its own on which we sit perched, and from which we look in two directions into time. The unit of composition of our perception of time is a duration, with a bow and a stern, as it were—a rearward- and a forward-looking end” (p. 609).
Edmund Husserl reached a similar conclusion, in a different way. I suspect that he and James were right, but their disagreement with Kant is relatively subtle. The main point is that mental models that represent time as a line, which we have developed to understand history and nature, do not reflect how we actually experience time.
- It may be possible to experience time differently
If a linear model of time is highly useful and embedded in our language and culture, we will never just drop it. But if it is contingent on our circumstances and fails to describe our own experience, then it is not exactly objective or obligatory. We may be able to think in different ways, at least at times.
The great Zen thinker Dogen (1200-1253 CE) worked out a theory in which time is not separate from being, a vessel or dimension or space in which things occur. Instead, being and time are the same. “The way the self arrays itself is the form of the entire world. See each thing in this entire world as a moment of time” (p. 92).
I am not sure that his abstract metaphysics persuades me or communicates what he perceived while he meditated on time. I get more from his verse (p. 177):
For thousands of yards, the cold lake soaks up the color of the sky.
Evening quiet: a fish of brocade scales reaches the bottom,
then flits this way and that; an arrow notch splits.
Endless water surface, moonlight brilliant.
Sources: Buddhaghosa, The Path of Purification, translated by Bhikkhu Nanamoli (Buddhist Publication Society, 2010), viii.7 (p. 476); James, The Principles of Psychology (Henry Holt, 1918); Kazuaki Tanahashi and Peter Levitt (editors and translators), The Essential Dogen: Writings of the Great Zen Master (Shambhala, 2013)
See also: how to think about the self (Buddhist and Kantian perspectives); phenomenology of nostalgia; A Husserlian meditation.