There’s an old saying: no person can dip his toe in the same river twice. It’s never the same river. Nor the same person.

There is a profound truth in this. I am not who I was as a five year old or even a twenty-five year old, and not even quite who I was yesterday. In the same way, a river’s water rushes constantly toward the sea, replaced constantly by fresh supplies from its source, perhaps hundreds of miles away.

And yet, I am also very much a continuous entity, as I’ve always been. The five year old lives on in me by virtue of everything she loved and learned and imagined. The imprints of memory on her mind are still etched in mine. Some form of her body is retained as mine, although all its cells have grown and aged–many have sloughed off and been replaced–in a way almost unrecognizably distinct from hers. Almost.

A river has a similar story. Every drop of water in it is perpetually renewed. The soil on its bed is washed away and rebuilt. Yet the course retains its continuity. In a very real sense, it certainly is the same river.

Speaking of continuity, I am sometimes awestruck by the rivers that flow through human settlements. I caught myself once casually thinking about one of the rivers of New York City as a feature of the city. The thought stopped me in my tracks. There it lies, among the buildings, bridges, highways, and promenades that have grown up all around it, almost engulfed in the swirl of humans and all the artifice of their making, just another shimmering jewel in this bejeweled city. But how odd to perceive it that way! The river, unlike everything else that I see as part of the city’s structure, is NOT a thing of our making. It is primeval. It has been flowing for thousands of years. It is not a feature of my city. My city, in all its glory, is just a transient bit of glitter that has briefly arisen around its banks and will shortly fall away, just as any bit of driftwood that rolls out to sea.