The nature of television, as opposed to a live performance, means it takes a long time to work out reactions you've caused. The first I hear is when my mum rings to tell me the lady over the road saw it and said it was very good, but that's of limited value, as the lady over the road was never likely to say "I saw your son on the telly last night and he was shit, Mrs. Steel."

Eventually you pick up from people who approach you whether it made an impact, which at first left me slightly bewildered. Because I'm a comedian, so after an enthusiastic speech from someone telling me the show explained the conflict between Freud and Jung, or clarified the theory of relativity, I'd have to stop myself from yelling "But what about the bloody jokes?" It seemed as peculiar as if someone told Michael Palin the parrot sketch was marvellous because up until then they had no idea there was a bird called a Norwegian Blue.

So it took a while to appreciate that if someone feels they understand an idea as a result of the programme, that's a success.

Of the subjects so far, who has been the hardest to boil down into a thirty-minute slot?

Of the subjects I've covered so far, the hardest was, not surprisingly, Einstein. I doubt whether it's possible to even begin to understand him without becoming obsessed by his theories. I wrote it in an office at Broadcasting House, and one day I embroiled several other people into my mania. At one point there were fourteen of us drawing diagrams and shouting questions such as "But why would the spaceman go slower?" so that the BBC management must have looked at their productivity figures and pondered why that day, not a single programme got made at all. That night I was sat in the bath with an Einstein book, and a sheet of paper covered in arrows, and heard my eight-year-old son shriek "Oh my God, Dad's in the bath talking to himself about the speed of light."

How do you choose the subjects for the lectures? Are you open to requests?

The process of choosing the subjects is not very scientific, and is always a calculated gamble, as I usually don't know a great deal about them before starting. It's a bit like the way I'd buy a car. Knowing nothing about mechanics, I wait until one, for some reason, feels right.

I suppose ideally the subject should be someone most people have heard of, but know little about. It's even better if there's an idea connected to them, such as natural selection or the Oedipus Complex, that most of us are aware of without knowing what they mean. Then it helps if they behaved in a magnificently unexpected way, so Descartes was perfect.

After reading one page of an introductory guide to the bloke, I thought a) I can't wait to find out what 'I think therefore I am' actually means, and b) Bloody hell, he did his work sitting in an OVEN.

Similarly, while Sylvia Pankhurst was an extraordinary figure, she sealed her place in history by accepting a position as virtual princess for Haile Selassie.

The other issue that makes someone, in my view, fascinating, is placing them in the context of their times. This can be the area that academics, obsessed by the minutae of their subject, miss out altogether. For example, they can be so concerned with interpreting a specific phrase in the Canterbury Tales that they ignore the enormous changes taking place in English society that made Chaucer possible. Leonardo da Vinci and Beethoven were geniuses, but they'd never have been heard of if they hadn't lived in revolutionary times that nurtured their genius. Tying these two elements together is what I find compelling, rather than dwelling on the tiny details of a painting or symphony.

Following on from that, in answer to whether there's anyone around now who could one day appear as fascinating as these characters, I suppose you'd have to apply the same rules. The most contemporary character I wrote about for the radio series was Muhammad Ali, who would have been one of the most extraordinary figures of the twentieth century even if he hadn't been world heavyweight boxing champion. Again, not only was he an amazing character but he personifies that most dramatic period of American history, dominated by the movements to stop the Vietnam War and for civil rights. Eras such as this, like the Renaissance or the French Revolution, seem to burst with astonishing characters, when anything seems possible and all that was assumed to be eternal is thrown into doubt. So Nelson Mandela is an obvious candidate, and in a different way so are Bob Dylan and Johnny Cash. And for completely different reasons, having gone from humiliating the US Senate to something as bizarre as going in the Big Brother House to shout "You are a lying plutocrat" at Preston, so is George Galloway.

All the lectures are of people you obviously have a degree of admiration for - have you been tempted to do a series of anti-heros? It's well spotted that all the characters in the programme are people I admire, at least a bit. But they are also all magnificently flawed. Chaplin and Einstein were dreadful to women, Beethoven was a foul-tampered bastard, Isaac Newton became a spy against the poorest people in the country and so on. This is what makes them fascinating, rather than the black and white world in which people are either goodies or baddies. How do you categorise Cromwell? It wasn't as if he was a Republican revolutionary who then became a slaughterer of the Irish and the radical movement, he was both at the same time.

But also, I think it's much more rewarding to write about someone who had a positive impact. Partly this is because the sub-text in the story of, for example, Tom Paine, is that all of us are capable of being little Tom Paines. We can't all write books that change the world but we can summon up the courage to oppose the racist or the manager who bullies the boy in the office. We can't all be Einstein but we can remember to do as he did and retain a sense of wonder at the stars.

Content last updated: 14/02/2006