Upon our return to Earth the newsplastics hailed me as one of the most highly reintegrated individuals anywhere. In all the teeming domain of man there were only seven hundred who had gone through as many substitutions as I had. Where, they philosophised in passing, would a man cease to be a man in the sequence of substitutions?

Philosophy had never been an important preoccupation of mine. It was the only discipline no further ahead in its really essential questions than the Greeks of four thousand years ago. Oh certainly, there had been lots of technical improvements that were fascinating but these were peripheral points; the basic issues could not be experimentally tested so they had to remain on the level of accepted or rejected axioms. I wasn't about to devote much time to them when the whole fascinating field of subatomic mirror numbers was just opening up; certainly not because a few sensational journalists were toying with dead-end notions. For that matter the newsplastics weren't either and quickly went back to the regular mathematical reportage they do so well.

A few decades later, however, I wasn't so cocksure. The old Centaurian virus had reappeared in my brain of all places and I started to have a peculiar feeling about where the end point in all this reintegrating routine would lie. Not that the brain operation was a risk; thousands of people had already gone through it and the substitute organisms had made no fundamental change in them. It didn't in my case either. But now I was more second matter than any man in history.

"It's the old question of Achilles' Ship," Dr. Erics told me.

"Never heard of it," I said.

"It's a parable, Treb, about concretised forms of a continuum in its discrete aspects."

"I see the theoretical question but what has Achilles' Ship to do with it?"

He furrowed his protoplast brow that looked as youthful as it had a century ago. "This ship consisted of several hundred planks, most of them forming the hull, some in the form of benches and oars and a mainmast. It served its primitive purpose well but eventually sprang a leak. Some of the hull planks had to be replaced after which it was as good as new. Another year of hard use brought further hull troubles and some more planks were removed for new ones. Then the mast collapsed and a new one was put in. After that the ship was in such good shape that it could outrace most of those just off the ways."

I had an uneasy feeling about where this parable was leading us but my mind shied away from the essential point and Erics went relentlessly on. "As the years passed more repairs were made—first a new set of oars, then some more planks, still newer oars, still more planks. Eventually Achilles, an unthinking man of action who still tried to be aware of what happened to the instruments of action he needed most, realized that not one splinter of the original ship remained. Was this, then, a new ship? At first he was inclined to say yes. But this only evoked the further question: when had it become the new ship? Was it when the last plank was replaced or when half had been? His confidently stated answer collapsed. Yet how could he say it was the old ship when everything about it was a substitution? The question was too much for him. When he came to Athens he turned the problem over to the wise men of that city, refusing ever to think about it again."

My mind was now in turmoil. "What," I demanded, "what did they decide?"

Erics frowned. "Nothing. They could not answer the question. Every available answer was equally right and proved every other right answer wrong. As you know, philosophy does not progress in its essentials. It merely continues to clarify what the problems are."

"I prefer to die next time!" I shouted. "I want to be a live human being or a dead one, not a machine."

"Maybe you won't be a machine. Nothing exactly like this has happened before to a living organic being."

I knew I had to be on my guard. What peculiar scheme was