MY FRIENDS are all very witty, as I’m sure you realise. Much wittier than yours. Anyway, one of them did this really, really funny thing once that made us all laugh and laugh.

There used to be a free-market libertarian bookshop in Covent Garden. It sold pamphlets on things such as how to cut crime by giving criminals tradeable vouchers. Or you could buy a T-shirt with a Warhol-style print of Friedrich von Hayek. You should have gone while you still could. It has closed down now — supply exceeded demand.

One day, as my friend was leaving after a happy half-hour browsing in the section on the marginal cost of free love or some such, the guy behind the counter shouted out: “Shut the door.” And my friend replied: “How much is it worth?” Brilliant, no? Well, perhaps it was the way I told it. But it amused me, and I’ll tell you why.

I used to enjoy the libertarian bookshop. Its shelves were stacked full of interesting ideas. And I believe in free markets and reducing the size of the State. It’s just that while I like all these things, I don’t like fanatics.

A couple of months ago saw the 100th anniversary of the birth of the American author Ayn Rand. You may have forgotten to buy in candles and a cake, but the centenary of this Russian immigrant is worth marking. And not only for the reasons she would have liked.

In 1943 Rand published a novel that was greeted with indifference. Called The Fountainhead, it garnered critical reviews for its clumsy prose, but its sales grew and grew. The same happened with its successor, Atlas Shrugged. Large sections of this vast book are almost unreadable, but millions of copies have been sold. In 1991 the Library of Congress conducted a survey to establish which books had made the greatest impact on the lives of readers. The Bible came top. Atlas Shrugged was second.

Why was this? Rand’s books were works of fiction, but their power came from their ideas. Rand advanced the cause of reason and personal freedom. Atlas Shrugged, in particular, is about the consequences when the jealousy and weakness of others stifles the creativity of the most talented.

For me, as for so many others, Atlas Shrugged was a convincing book, an inspiration. Which is odd, because I’m equally convinced that Rand was a nutter.

Out of the college Ayn Rand clubs and groups of devoted readers, Rand created a cult in which only those who agreed with her on everything, right down to her musical taste, were regarded as intelligent enough to understand her true philosophy.

She also anointed an ideological heir, Nathaniel Branden, 25 years her junior. Then she had an affair with him. Both her husband and Mrs Branden were expected to understand (and, it appears, did) because a relationship between two such great thinkers was “rational”. Then, when Nathaniel Branden started another affair, Rand erupted in fury and split her movement by denouncing both Brandens and expelling them (bit unfair on Mrs Branden, but there you go).

So Rand’s centenary should be marked because it celebrates the birth of one of the inspirers of the late 20th-century revival of liberty and freedom. And it should also be marked because her life story shows the dangers of extremism and certainty.

Even those who advance the idea that nothing is sacred except reason, even those who believe that every institution must be open to challenge from better ideas and better people, can become dictatorial figures immune to either reason or challenge.

Freedom and liberty need great champions. But so do moderation and compromise.

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