I am trying to reduce a thousand years into a comprehensible chunk of time, and am finding it surprisingly easy.

Some very old people, after all, will have witnessed more than 10 per cent of this millennium single-handedly (a very old giant tortoise in the Seychelles will have witnessed nearly a quarter of it). In theory, this means that it will have taken a mere 10 people to witness the whole thousand years.

Make that 20 and you have got human history safely covered right back to the Immaculate Conception.

What is a millennium, after all, but a handful of centuries or a few dozen decades? Looking ahead, it is actually quite easy to imagine that the great- grandfather of a giant tortoise destined to witness the dawn of the fourth millennium has already been born.

Given the likelihood of developments in genetic engineering leading to greatly lengthened - if not eternal - human lifespans in the relatively near future, it is even possible that somewhere, probably in California, a great-grandfather of a human being destined to witness the dawn of the fourth millennium has already been born.

Not convinced? We often talk about the extraordinary changes that have occurred in our lifetimes, about the vast cultural and spiritual chasm that separates us from those who lived 2,000 years ago. But I am convinced that we overstate these. Imagine getting 20 old people together, each representing a century from the last 2,000 years. It would be no different from putting a British beef farmer into a minibus along with, say, a French truck driver, a Chechen rebel and a German footballer.

In other words, it would not take more than a minute before people started making disrespectful comments about each others' centuries, affectionate or otherwise. Stereotyping ("typical 18th century"), prejudice ("the ninth century was a dump") and chauvinism ("there is no other century remotely like ours") would soon roll along, once familiarity had bred the appropriate degree of contempt.

Everybody would profess loud disappointment. The middle centuries would disdain to talk to early centuries. Early centuries would reject the notion that the passage of time should be equated with progress. At least one of them would be a total maverick, refusing to hold session with the others.

Every century from the beginning of the Renaissance in around 1400 onwards would trumpet its own cultural superiority, dismissing previous centuries as barbarian and subsequent ones as parvenu. There would be much talk of dwarfs standing on the shoulders of giants. It would be like a summit of world leaders, with the 20th century as Bill Clinton.

In other words, it would be like 20 old people in a minibus. The mystery of elapsed time would drop away, and 2,000 years would seem like one rather hectic day.