During my recent trip to Boston I visited my old territory in Cambridge, and I walked down the streets that had been my path to Harvard Square for ten years. The route took me through the back lots of the biological science buildings, past the Agassiz Museum with its famous glass flowers and mineral specimens, and down Oxford Street until I reached the Science Center. This modernist edifice houses the undergraduate science and mathematics classrooms and labs.

The sidewalks of Oxford Street are paved with red bricks which match the famous "crimson" bricks of Harvard's historic buildings. They are old bricks, perhaps from the nineteenth century, and over the years they have gotten quite uneven from frost, differing pressure, tree roots, and construction projects. I had to keep my eyes on the path in order not to trip up on one of these projecting bricks, lest I trip and fall onto a hard icy surface.

One of my gifts, and my curses as well, is that I notice everything. The world is full of myriad details, all calling out to me. I don't easily tune anything out, which is one reason why I hate the noisy soundtracks in stores and restaurants. And thus with the red brick path, I noticed irregularities in these antique bricks, whether they were fashioned by people or by the circumstances of Cambridge weather. Some bricks had pits eroded by ice and sand, others were broken in place, and still others were worn down by a century of traffic. And other pavers of this antique walkway still bore the marks of the brickmakers, such as stamps or letters or other identifications, from makers long gone.

One of the bricks which I passed nearly every day was in the shadow of the Science Center. It had what I assumed was a maker's mark, which was not stamped or pressed into it but inscribed by hand while the brick was still soft and unbaked. It was in the shape of an infinity, the characteristic horizontal figure-eight of mathematics. Perhaps it was really an "eight," for after all with brick marks there is no preferential direction. But I chose to consider it an infinity, which brought to my mind even then, notions of mathematics and science practiced in the building next to it. I called it the "Infinity Brick."

I looked for the Infinity Brick every time I walked down that street. It was a reminder to me of a cosmic order that was greater than Cambridge, perhaps even greater than Harvard. I could tread on infinity, encoded into a brick as durable as anything from ancient Babylon.

Now that I know a bit about calculus, I can see that approaching and then moving away from the Infinity Brick was like moving mathematically toward and away from a limit. Just as with the tortoise of the famous paradox, I could approach the Infinity Brick in an infinite division of distance or time, and yet still get there. Then once I had gone over the Brick, I went toward the negative infinity of Harvard Yard. When I was first introduced to limits, with their traveling point on a graph, I wondered what that point "felt." It was a living thing to me, experiencing forces. And then, if the limit plummeted or rose to infinity, the conscious point was shot out of reality into a pointless netherworld. What is it like to cross the limit? Is there a place where everything disappears until your graph becomes sensible again? How do you keep from winking out of existence when the denominator gets to zero, even if it never…quite…

I say that you keep from winking out of existence at the limit by the use of memory, either your own or someone else's, or by the inscription of memory on some long-lasting medium. The discontinuity disappears if I remember where I am going. In the case of Cambridge, I had not forgotten anything, including the location of the Infinity Brick.

So I anticipating meeting again with my inscribed brick by the Science Center, to tell it how I now understood its position as a calculus limit marker. But when I got to the walkway where the Brick would be, I found to my dismay a brand new walkway, composed of Harvard red bricks that were not old, tilted, or pitted, but were flat, mechanically rectangular, and featureless. The old bricks, including the Infinity Brick, were gone. What a disappointment! Where were they now? Had the old bricks just been tossed out into some landfill, or had they been re-cycled by luxury builders into some "antique brick" structure? My durable baked clay of memory was no longer there to mark the passage. Perhaps there is some other place, another walkway or wall, where the Infinity Brick's calculus goes unnoticed as it slowly becomes obscured by ivy and the shadows of time.

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