The most commonly used noun in American English is time. But if you ask a scientist who studies time to explain what time is, he or she invariably will turn the question on you: “What do you mean by time?”

And already you’ve learned something. You might begin, as I did, by qualifying your statement to mean “time perception,” to distinguish between external time and your internal grasp of it. This dichotomy suggests a hierarchy of truth. Foremost is time as told by one’s wristwatch or the clock on the wall, which we typically think of as “true time” or “the actual time.” Then follows our perception of this time, which is accurate or not depending on how closely it matches the mechanical clock. I’ve come to think that this dichotomy is, if not meaningless, certainly of little help in trying to understand on a human scale where time comes from and where it goes.

But I’m jumping ahead. One of the oldest debates in the scientific literature is whether “time” is something that can be “perceived” at all. Most psychologists and neuroscientists have come around to thinking that it isn’t. Our five senses — taste, touch, smell, sight, and hearing — all involve discrete organs that detect discrete phenomena: sound is what we call it when vibrating air molecules trigger movements of the tympanum in the inner ear; sight is what results when photons of light strike specialized nerve cells at the back of the eye. In contrast, the human body has no single organ devoted to sensing time. The average person can sense the difference between a sound lasting three seconds and another sound lasting five seconds, as can dogs, rats, and most laboratory animals. Yet scientists still struggle to explain how the animal brain tracks and measures time on so fine a scale.

One key to understanding what time is, physiologically, is to know that when we talk about time, we may be referring to any of a number of distinct experiences, including

Duration — the ability to determine how much time has elapsed between two specific events or to accurately estimate when the next event will occur.

Temporal order — the ability to discern the sequence in which events occurred.

Tense — the ability to discriminate between the past, present, and future, and the understanding that tomorrow lies in a different temporal direction than yesterday.

The “feeling of nowness”—the subjective sense of time passing through us “right now,” whatever that is.

Suffice to say, discussions of time often get confusing because we’re using just one word to describe a multilayered experience; to the scientific connoisseur, time is as generic a noun as wine. Many of these temporal experiences — duration, tense, simultaneity—feel so basic and innate that they hardly seem to merit distinction. But that’s only true from an adult perspective. The view in developmental psychology is that time is something that humans come to know only gradually. One fundamental insight comes to us in the first few months of life, when we learn to distinguish “now” from “not now” — although the seeds of this awareness probably reach us even sooner, while we’re still in the womb. Not until age four or so can children accurately distinguish “before” from “after.” And as we age, we become ever more keenly aware of the “arrow of time” and its unidirectional flight path. Our knowledge of time is hardly as a priori as Kant proposed. Not only is time something that gets in us, it takes years to fully do so.

This is an extract from Alan Burdick's Why Time Flies published by Simon & Schuster