I FOUND MYSELF in the Natural History Museum the other day, watching a film about meteorites. For centuries, the narrator intoned, the iron found in them was the strongest material available to humankind. Oh, I thought, without a second's hesitation, right: just like Iorek Byrnisson's armour. Of course.

Iorek Byrnisson, as many of you will know, is Philip Pullman's creation – the rancorous, heroic armoured bear who first appears in Northern Lights. “Armoured bear”? Yes, what else? His protection forged from “sky iron”, his ferocity guaranteed. As Amanda Craig says in her piece on the durability of imagined worlds this week, it is no small feat to conjure a place and its inhabitants with such vividness that a woman in a museum in the real world will draw useful parallels between the two.

That assuredly is what Pullman has done, his other Oxford joining J.R.R. Tolkien's Middle-earth, Ursula LeGuin's Earthsea and C.S. Lewis's Narnia as places whose existence may not be called into question, though they exist – so they tell us – only in the minds of their authors and readers.

It's hard to put your finger on what makes an imaginary place real — but, as with any landscape you love, you know it when you see it. This makes me somewhat nervous about encountering Hollywood's version of Lyra's Oxford, although no doubt I'll go along. The truth is that I don't need to see the film of The Golden Compass (as the cinema version of Northern Lights is called), for I've seen it many times before in my head. I know what Lord Asriel looks like and it isn't Daniel Craig. But that doesn't matter— the wonder of the internal film created by author and reader together is its durability. I quite liked the recent film of The Lion, the Witch and the Wardrobe; but I can't remember anything about it, really. When I think of the White Witch, or Edmund in the snow with that Turkish delight, the image is one I've had in my mind since I first read the book 30 years ago.

I'm also moved to note that if you like coincidence, as I do, you may be pleased to find an extract this week of the late Norman Mailer's final book, On God. Pullman, in His Dark Materials, like Mailer, is bold enough to take on God; and it's perhaps surprising that Mailer — that literary bruiser — seems gentler in his judgment of religion than Pullman can be. But Norman always was full of surprises — for which we must give thanks.

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