Which writers do you consider exceptionally intelligent? I realize that intelligence is hard to define, but don't find it hard to recognize. I am looking for those I have not yet discovered, and I would rather not influence this thread by writing my own list. All genres and time periods welcome.

Just the definition would open a can of worms. But I felt I was in the presence of a brilliant mind when reading 'Revenge of Gaia' by James Lovelock. And it was so enthralling it read like a work of fiction, while being solidly real. His latest book was published earlier this year. I've no doubt it's just as gripping but I haven't read it yet. Nigel

Edmund Spenser, Terry Pratchett, Neil Stephenson, Charles Stross, Douglas Adams, Mary Gentle, Neil Gaiman, Alan Moore, Robert Anton Wilson, John Brunner. All authors who have an extremely wide breadth of knowledge, combined with a deep understanding of humanity and the gift of being able to throw complex ideas and concepts into their work without fear or favour or dumbing down,(and quite often with humour) thereby increasing the knowledge base and intellectual scope of their readers without beating them over the head whilst entertaining them with rich storytelling.

Henry James and Shakepseare of course, but dare I mention Thomas Moore? People read Utopia and a lot don't seem to get the sheer wit and genius that is shown in this story.

Chaucer.

Robertson Davies - the Canadian author comes across as the sort of amusing intellectual who would have made him a great dinner guest.

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Would you be so kind as to explain yourself, E. Ward? Someone seems to think that your question mark "adds to the discussion", but I don't see how it does. Did you not understand the question? I am indebted to those who have responded with something other than punctuation, as I am not familiar with most of those named.

She didn't write fiction (though much of her writing could at a stretch be called 'literary'), so perhaps this isn't a suitable answer: Simone Weil. Without a doubt.

Kurt Vonnegut is an exceptionally incisive and humane writer, though his unconventional narrative style may not be to all tastes. JG Ballard likewise.

The most perceptive and intelligent book I've read recently was 'Education, Edukation, Edukashun,' by James Rainsford. It was brilliant and was obviously written by someone both immensely observant and talented. You can read reviews here: Education, Edukation, Edukashun

I would say Saul Bellow. Some of his novels contain such detailed references to umpteen great thinkers, many of whom I hadn't heard of and still know little about. But he is certainly one author whose work radiates intelligence, and in certain books - "Herzog" for example - the reader can find themselves following details of people they know nothing about through the context. I would also say that I found it a little intimidating at first.

As a writer myself, I can honestly say that my own work does not quite exude such learning, as anyone who follows the link below will be able to attest, although hopefully they'll find it funny.

Sherlock Holmes and the Underpants of Death

Mr W, I definitely agree about Saul Bellow. I'd also add Thomas Mann, Anthony Powell, and Thomas Keneally to the list. Another obvious addition is Umberto Eco, whose fiction is even more overtly intellectual than Bellow's.

Dostoyevsky without a doubt

Neal Stephenson is one smart guy, as can be seen by the shear breadth of ideas in his books.

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I too must agree about Bellow, especially Herzog. And even though a lot of people don't like them so much, his later novels such as The Dean's December, which was greeted with a lukewarm response to say the least, are still evidence of a great mind at work.
Also Beckett - and I'm thinking not so much of his drama as of his wonderful early prose, particularly Murphy - absolute genius

apart from Shakespeare?

Richard Dawkins writes intelligent stuff - even if you ignore his God Delusion, works like The Selfish Gene are entertaining, informative and make u think

Very high intelligence is a serious handicap in most walks of life, including authorship. Mensa members are reduced to writing How To books for which they're generally expected 'to make a contribution towards the cost of production.' Same applies to anthologies of writing by Mensa members. Exceptions to the rule are condemned to obscurity anyway. (I know whereof I speak!) The jungle has become very thick.
In their book reviews section, Amazon.co.uk very effectively gag intelligence.

"Very high intelligence is a serious handicap in most walks of life, including authorship."

You exaggerate. Very low intelligence is much more of a handicap.

Writing is an art and no one claims that very intelligent people make better artists. Perhaps highly intelligent people spend too much time doing what they are good at and neglect other parts of their brains. Hence lose the capacity to write well. However I expect it is largely inate.

Richard Powers. Writes superb "big" novels, and appears to have a brain the size of a planet.

Dawkins & Steve Jones are bright lads too, fair play.

I don't really see your point, BWE. What does Mensa have to do with intelligence? Did you think I was talking about people like Stephen Hawking? Although mathematical prowess, a good memory and trade skills all require a certain degree of intelligence, none of them define it, just as no test could ever define it.
Would you care to explain your comment about the review section?

How do they "very effectively gag intelligence" then?

A S Byatt - especially in her book Babel Tower
Edward O Wilson and Bert Holldobbler (I might not have spelt that right) (they wrote a lot about the social lives of ants and made it exciting)
Philip Roth - mostly early work sad to say
Beckett - Murphy as another person above has said
Joyce - Ullysses
Penelope Fitzgerald can say more in three paragraphs than most can say in a chapter.

You ask 'What does Mensa have to do with intelligence?' I'll spare you any categorisation of the question. Intelligence is measured by IQ tests, not by the recommendations of book reviewers (which is not to say that the recommendations are necessarily worthless.)
Few people with exceptional intelligence - your subject - have very much in the way of 'mathematical prowess', good memory or 'trade skills', whatever they're meant to be! You seem to have very curious ideas about what you claim to recognise but can't define.
You ask me to explain my comment about the Amazon reviews section. So does E. Shaw.
One-star reviews from me are now rejected by Amazon.co.uk They decline to publish! I'm black listed! Non-automated explanations from them come very close to clinical dyslexia. Try to point out that the plural of asterisk is not asterisk's, and they repeat their refusal to publish anything which uses asterisk's to mask profanity. (The profanity is in the books reviewed - one of which is described on its own dust jacket as 'bitterly intelligent' - the asterisks are reported speech sanitised by me in the interests of decency.) The question to ask is, why should people who can't communicate be allowed the right to stop people who can from doing so? I've been a professional writer for almost half a cerntury now and know how to communicate. My last book was nominated for two major literary awards; it sank without trace (partly through gratuitous mention of Mensa - against my will - on the inside back cover. I know the scale of anti-intelligence amongst people who can't define it.)