Illustration by Miguel Porlan

Some nights—more than I like, lately—I wake to the sound of the bedside clock. The room is dark, without detail, and it expands in such a way that it seems as if I’m outdoors, under an empty sky, or underground, in a cavern. I might be falling through space. I might be dreaming. I could be dead. Only the clock moves, its tick steady, unhurried. At these moments I have the most chilling understanding that time moves in only one direction.

I’m tempted to look at the clock, but I already know that it’s the same time it always is: 4 A.M., or 4:10 A.M., or once, for a disconcerting stretch of days, 4:27 A.M. Even without looking, I could deduce the time from the ping of the bedroom radiator gathering steam in winter or the infrequency of the cars passing by on the street outside.

In 1917, the psychologist Edwin G. Boring and his wife, Lucy, described an experiment in which they woke people at intervals to see if they knew what time it was; the average estimate was accurate to within fifty minutes, although almost everyone thought it was later than it actually was. They found that subjects were relying on internal or external signals: their degree of sleepiness or indigestion (“The dark brown taste in your mouth is never bad when you have been asleep only a short time”), the moonlight, “bladder cues,” the sounds of cars or roosters. “When a man is asleep, he has in a circle round him the chain of the hours, the sequence of the years, the order of the heavenly bodies,” Proust wrote. “Instinctively he consults them when he awakes, and in an instant reads off his own position on the earth’s surface and the time that has elapsed during his slumbers.”

It may also be a simple matter of induction: it was 4:27 A.M. when I last woke at whatever hour this is, so that’s what time it is now. The surprise is that I can be so consistent. William James wrote, “All my life I have been struck by the accuracy with which I will wake at the same exact minute night after night and morning after morning.” Most likely it’s the work of the circadian clocks, which, embedded in the DNA of my every cell, regulate my physiology over a twenty-four-hour period. At 4:27 A.M., I’m most aware of being at the service of something; there is a machine in me, or I am a ghost in it.

And, once the ghost gets thinking, there is much to think about—most of all, how little time I have in which to do all the things I’m thinking about and how behind I am. Until very recently, that included a book about, of all things, the biology and perception of time, which had preoccupied me since before my kids—twin boys, Leo and Joshua, now ten—were born. In its wake is everything else: the melting ice caps; the cost of orthodontics; the rise of demagoguery; the gutters I have to clean before winter, if winter really comes. The end of the year is nearly here, and still my schedule is scattered across four productivity apps.

As worried as I am in these waking moments, I also find them oddly calming. It’s as if in falling asleep I’d fallen into an egg and woken as the yolk, cushioned and aloft on an extended present. It won’t last, I know. In the morning, the hours and minutes will reassert themselves and this seemingly limitless breadth of time will seem unreal and unreachable—the dream of boundless time, dreamed from the confines of an egg carton. But that’s a thought for tomorrow. For now, it’s now, and the tick of the bedside clock is the muffled beat of a heart.

For more than two thousand years, the world’s great minds have argued about the essence of time. Is it finite or infinite? Does it flow like a river or is it granular, proceeding in small bits, like sand trickling through an hourglass? And what is the present? Is now an indivisible instant, a line of vapor between the past and the future? Or is it an instant that can be measured—and, if so, how long is it? And what lies between the instants? “The instant, this strange nature, is something inserted between motion and rest, and it is in no time at all,” Plato remarked in the fourth century B.C.E. “But into it and from it what is moved changes to being at rest, and what is at rest to being moved.”

For St. Augustine, writing his “Confessions” in the year 397, time was even simpler: it’s us. Augustine was forty-three, beginning his tenure as an overwhelmed bishop in Hippo, a port city in North Africa, during the decline of the Roman Empire. The literature on time perception generally begins with Augustine, because he was the first to talk about time as an internal experience—to ask what time is by exploring how it feels to inhabit it. Time may seem slippery and maddeningly abstract, but it’s also deeply intimate, infusing our every word and gesture. Its essence, Augustine argued, can be gleaned from a single line of speech: “Deus creator omnium.”

“God, creator of all things.” Say it aloud or listen: in Latin, eight syllables, alternating short and long. “Each of these latter lasts twice as long as each of the former,” Augustine wrote. “I have only to pronounce the line to report that this is the case.” Yet how do we manage to make this measurement? The line is composed of syllables that the mind encounters in succession, one by one. How can the listener consider two syllables at once to compare their durations? How can one hold the longer syllable in mind? Its duration can’t be defined until it’s completed, but by then both syllables are gone. “Both have made their sound, and flown away, and passed by, and exist no more,” Augustine wrote, asking, “So what now exists for me to measure?”

Here Augustine arrived at an insight so fundamental that it’s taken as a given: time is a property of the mind. When you ask yourself whether one syllable lasts longer than another, you aren’t measuring the syllables themselves (which no longer exist) but something in your memory, “something fixed and permanent there.” The syllables leave an impression that persists in the present. Indeed, Augustine wrote, what we call three tenses are only one. Past, present, and future are all immediate in the mind—our current memory, our current attention, our current expectations. “There are three tenses or times: the present of past things, the present of present things, and the present of future things.” Augustine plucked time from the realm of physics and placed it squarely in what we now call psychology. “In you, my mind, I measure time,” he wrote. Words, sounds, and events come and go, but their passage leaves an impression: “Either time is this impression, or what I measure is not time.”

To consider this present is to glimpse the soul, Augustine argued. Modern science has abandoned the soul in favor of probing the framework of consciousness, a concept that is only slightly less elusive. Yet we share a rough idea of what’s meant: a lasting awareness of one’s self moving in a sea of selves, dependent yet alone, or a deep and common wish that “I” somehow belong to “we,” and that “we” belong to something even larger and less comprehensible; and the recurring thought, so easy to brush aside in the daily effort to get through our to-do lists, that our time matters precisely because it ends.