The other day I went for a stroll along the shore here in Wellfleet. The Outer Cape is very nearly the easternmost extension of the continental United States (save for a stretch of coastline in eastern Maine), and as I stood facing the sea I was aware of standing precisely on the boundary of two vastnesses: the great and stormy Atlantic before me, with the whole American continent stretching out behind.

But there was more to it than that: I stood also on the exact limit of a vertical frontier. Beginning precisely at the soles of my feet was nearly eight thousand miles of solid Earth, while above me the thin blue sky soon gave way to infinite and empty space.

As I thought about this I remembered that there was still another sense in which we all stand upon an edge, a boundary, a frontier: our occupation of the mysterious place that we call the present. It is always vanishing, but always with us; it is infinitesimally small, but somehow it is where everything happens. Behind us, like the vast American continent, is the past; it is, like the land itself, “written in stone”. Ahead is the future, as teeming and unknowable as the ocean.