I had been walking a long time with a box. Over the weeks and months that I carried this box, it grew heavy on my arms and many times I stopped to look what was inside. Yet upon opening the four flaps, I saw nothing inside but some dust. Each time, I refolded the four flaps, mounted the box on one shoulder, and continued walking.

After some months of walking I found there was a pain in my shoulder which I couldn't avoid thinking about: a sharp soreness that pressed itself on my mind. I stopped in the next village and found a doctor. He examined my arm and pronounced a harsh diagnosis: surely my arm would fall off at the shoulder within the next six months. He continued examining my arm and then took a curious expression on his face. Handing me a baseball, he led me out into the courtyard, walked several dozen yards away, and asked me to throw.

I threw.

He fielded the baseball and tossed it back to me. He now seemed more determined and more curious to ascertain some unknown fact from this demonstration; his eyebrows were peaked. "Again, faster!" he called out.

I threw the ball again, faster and this time taking more precise aim, using my sore arm as I did the first time. This caused me some pain but it was only like the pain of a paper-cut in your finger, when you are writing with a pen—a liminal buzzing noise.

He caught the ball midair and tossed it back. "As fast as you can!" he shouted in his learned, foreign accent. "Drive it!"

I threw the ball, as fast as I could. This time it came so fast that the doctor stepped aside, afraid of catching it, and let it hit a brick wall behind him, where it chipped off a sharp red fragment of brick. He fetched it quickly and came running to me.

"It is as I expected. Forgive my circumambulations. You see, I was once the staff doctor for the great baseball team, C____ B____s. In examining your arm, I noted certain structural irregularities which I have seen only once before, in the star pitcher of the B____s. I had to check my conclusions, which I'm pleased to say proved correct. Yes, undoubtedly you could have been a supreme pitcher, amongst the top four or five greatest players who have ever pitched. It's a shame that you'll be losing your arm."

We broke open the box. Inside, as before, was only dust. The doctor, nonplussed, carried this box to a rusty apparatus operated by means of a sliding weight on a rigid arm. By tweaking the position of the weight one way and then the other, he was able to discover a precise balance between the gravity of the box and the weighted arm. Reasoning that the effective balancing force of the weighted arm would increase in proportion to the weight's distance from the pivot, he deduced the mass of my box: 74 kg.

"How could that be?" I asked. "It is a cardboard box, and there is nothing inside but dust?"

Wanting, as much as I, to know the answer, the good doctor carefully emptied the box into a larger receptacle, and scrubbed the inside of the box to remove traces of dust. Then he applied the apparatus again to determine the weight of the box without the dust: less than one kilogram. I was mystified, as if lost in a thick mist, but the good doctor in his plaid coat had a quick explanation.

"You see," said the doctor, "A bit of dust weighs nothing—or almost nothing. But your little box had so many small pieces of dust that altogether these nothings amounted to a great weight, which you carried for a long time." At this point, I stood back from the man with his learned, foreign accent, and turned and sat down in a dusty upholstered armchair where the window favored it. Inerted thus, I began to take in the man's words. "Ah, you see. By carrying so much nothing for so far you have become this." He paused and scratched his pockmarked forehead. "I will go into my study now. Sit here as long as you like."