“What is time? Things change, and time is how we keep track,” science writer James Gleick wrote in the final pages of his invigorating tour of our temporal imagination. Central to Gleick’s inquiry into our relationship with time is the observation that even humanity’s greatest thinkers, be the scientists or philosophers or poets, have failed to offer an adequate definition of what time actually is, producing instead a variety of aphorisms, wisecracks, and other clever evasions. (Susan Sontag, riffing on John Archibald Wheeler: “Time exists in order that everything doesn’t happen all at once.” Richard Feynman: “Time is what happens when nothing else happens.” Augustine: “What then is time? If no one asks me, I know. If I wish to explain it to one that asks, I know not.”)

Perhaps Borges put it best in his exquisite “refutation of time”:

Time is the substance I am made of. Time is a river which sweeps me along, but I am the river; it is a tiger which destroys me, but I am the tiger; it is a fire which consumes me, but I am the fire.

Still, despite how central time is to our experience of reality — it colors our psychology, it makes the fabric of life elastic, and it locates our thinking ego — we have failed to make our language and logic cohere around it.

When Gleick was a boy of six, his mother, Beth Youman Gleick, gave her own answer to this perennially slippery question in Time Is When — a charming children’s book exploring one of the first external experiences of which we are aware: the substance and passage of time.

The original 1960 edition, now nearly impossible to find, features marvelous vintage artwork by sculptor and illustrator Harvey Weiss, who belonged to the same circle of Connecticut author and artist friends as Ruth Krauss and Maurice Sendak. In 2008, the book was reprinted in a new edition with different illustrations, but the original — of which I was fortunate enough to track down a surviving copy — remains singularly scrumptious. Weiss’s drawings offer the perfect visual counterpart to the limber curiosity and elegant simplicity with which Gleick tackles one of the greatest complexities of existence by illustrating how the fragments in which we experience time — parts of the moment, parts of the day, parts of the year, parts of a lifetime — shape the nature and texture of our experience.

For an Eastern counterpart, see this uncommonly beautiful and subtle Japanese pop-up book about time and the cycle of life, then revisit Bertrand Russell on the nature of time, Sarah Manguso on its confounding and comforting ongoingness, and Gaston Bachelard on our paradoxical experience of it.