There is a curious phenomenon in academic life whereby, quite independently, several different investigators start work on the same problem at the same time. It happened in a particularly striking way in the world of Shakespearean studies a generation ago, when - simultaneously and serendipitously - five different scholars re-examined the many variants between the two early printed texts of King Lear and all came to the same conclusion. The answer was not printing-house error, as textual orthodoxy had long assumed, but theatrical revision. The two texts represented two different moments in the stage life of the play. Thanks to this development, the surviving "Quarto" and "Folio" printings of William Shakespeare's work are now regarded less as the author's last word than as working scripts, snapshots from an evolving theatrical journey.

There has never been a shortage of new books on Shakespeare, but it is hard to remember such a similar-sounding clutch of titles as the following, all of which have hit the library shelves in the past few years: Shakespeare's Philosophy: Discovering the Meaning behind the Plays (2006), by Colin McGinn; Shakespeare Thinking (2007), by Philip Davis; Shakespeare the Thinker (2007), by the late A.D. Nuttall; Shakespeare's Ideas: More Things in Heaven and Earth (2008), by David Bevington; the collection Thinking with Shakespeare: Comparative and Interdisciplinary Essays (2007), edited by William Poole and Richard Scholar - not to mention my own Soul of the Age: The Life, Mind and World of William Shakespeare (2008), which has just appeared in the US with the subtitle A Biography of the Mind of William Shakespeare. Each of these books begins from a different place but ends in a similar one - the image of Shakespeare as an elusive, myriad-minded thinker.

McGinn, professor and Cooper fellow at the University of Miami, is a professional philosopher. He reads Shakespeare's plays for the light they cast on some of the questions that preoccupy philosophers: "scepticism and the possibility of human knowledge; the nature of the self and personal identity; the understanding of causation; the existence and nature of evil; the formative power of language".

Nuttall was a classically trained critic at the University of Oxford. His posthumously published book reads Shakespeare as "the philosopher of human possibility" - empirical, flexible, anti-theoretical.

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Meanwhile, Davis, professor in the School of English at the University of Liverpool, proposes that Shakespeare's poetry functions like a "Renaissance brain scanner": his line-endings are "a form of slow-motion eye-map for actors' voices that offer deep insight into the workings of the human brain". This may sound fanciful, but Davis is such a gifted close reader of the ebb and flow of Shakespearean language that he persuades us he is really on to something.

The pauses, loops and transitions in the language of Hamlet, King Lear and Macbeth dramatise the process of making connections through which the human brain operates. Empirical research undertaken by Davis and a team of cognitive scientists is beginning to show that reading or listening to Shakespeare stimulates the brain into unusually vigorous activity.

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Bevington, emeritus professor in the humanities at the University of Chicago and perhaps America's most senior Shakespearean scholar, offers more orthodox fare, beginning with the Bard's ideas regarding those hardy perennials that you are not supposed to talk about at dinner parties: sex, politics and religion. He then examines the plays' attitudes to scepticism and Stoicism, before concluding with what he takes to be Shakespeare's "credo", which turns out to be a somewhat cosy combination of conservative politics and liberal humanism.

The puzzling thing about these various attempts to make Shakespeare into a distinctively modern thinker is how little they have to say about the forces that shaped Shakespeare's mind: his education, his reading, the philosophical commonplaces of his time. How much did Shakespeare really know about philosophy? Whom did he resemble in the style of his thinking? What was his attitude to, say, the "neo-Stoic revival" of the late 16th century, about which historians have written so eloquently in recent years? These are questions to which there are verifiable answers.

On the question of Shakespeare's style of thinking, Davis comes closest by way of a penetrating passage comparing him to Michel de Montaigne, the 16th-century French essayist. McGinn also gestures briefly towards Montaigne, but Nuttall offers no more than a glancing allusion to this key figure.

Yet it is abundantly clear from Shakespeare's linguistic borrowings, via John Florio's English translation of Montaigne (1603), that the Frenchman's essays shaped much of his most profound thinking - in King Lear and The Tempest especially - about knowledge and scepticism, nature and nurture, emotion and reason, and the centrality of sexual desire to human experience.

In Much Ado about Nothing, Leonato is distraught that the honour of his daughter and his family name have been besmirched by Claudio's accusation of sexual infidelity. He refuses to listen to the "counsel" of his brother: passion overrides rational advice. People always advise those who are in sorrow that they should show "patience", but they're not able to apply the advice when they are suffering themselves.

Leonato says: "I will be flesh and blood,/For there was never yet philosopher/That could endure the toothache patiently,/However they have writ the style of gods,/And made a push at chance and sufferance."

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The irony here is specifically directed against Stoicism, the ancient philosophy that preaches "patience" in the face of adversity and recommends that we try not to be influenced by fortune and its reversals, aspiring instead to be like the gods, who are above suffering. A testing of the limits of Stoicism is one of the principal motifs of Shakespeare's Roman plays. If there is one thing we may say for sure about Shakespeare's philosophical position, it is that he was an anti-Stoic.

That was because he knew that we have bodies as well as minds. A man who makes his living from the passions of the drama and the physicality of the actor's body on stage is unlikely to have much patience with Stoicism's faith in the resolute mind alone. There was, however, an alternative ancient philosophy that gave due place to the body and its needs.

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In one of Montaigne's essays, immediately after a passage quoted by Prospero in The Tempest, there is a defence of Epicurus, a figure who was much maligned in the Renaissance because of his atheism and his lax attitude towards bodily desires. Neither Montaigne nor Shakespeare shared the atheism, but they were drawn to Epicurus' openness to all experience. In the course of his reading of Montaigne, Shakespeare encountered Epicurus' acknowledgment that desire and sensuality are essential parts of what it is to be human, his questioning of the Stoical faith in the stiff upper lip, and his resistance to the pursuit of public glory and posthumous fame - the latter idea summed up in the Epicurean precept that would have been the perfect motto for Shakespeare: "Hide thy life."

Aristotle said that participation in civic life is what defines the human condition. Late in the 4th century BC, Epicurus set up his "Garden School" on the outskirts of Athens with the express purpose of teaching the opposite.

In a wonderful book called Gardens: An Essay on the Human Condition (2008), the critic Robert Pogue Harrison points out that whereas Plato's Academy and Aristotle's Lyceum contained gymnasia that were located on public grounds, Epicurus' garden was private property.

In this it "reflected one of the pillars of Epicurean philosophy: the affirmation of what Pericles had called idiocy, by which he meant apoliticism, or keeping to oneself. Epicurus was in fact a militant idiot who thought of his garden as a haven from public life... as a thinker, Epicurus distinguished himself from the mainstream of Greek thought by depoliticising the concept of happiness and dissociating it from its traditional link to citizenship". Locating his school in a garden was the key to this.

Montaigne retreated from the French court to read books in his tower and cultivate vegetables in his garden. While Shakespeare's contemporaries pressed for preferment in the fevered world of the Jacobean court, and frequently got themselves into trouble for meddling with politics, Shakespeare kept his counsel and retired - possibly a great deal earlier than most biographers imagine - to his garden at New Place in Stratford-upon-Avon. In this, the pattern of his life, like Montaigne's, was Epicurean.

I am one of those people who sees what a book I am writing is really about only when I have finished it. Soul of the Age ended with some brief speculations about Shakespeare the Epicurean, but I now think I severely underplayed my hand. Everywhere I look - especially in the second half of his career, after he read Montaigne - I see links and echoes. A play such as Measure for Measure, with its obsessive interest in mortality and sexuality, makes sense when seen in an Epicurean light.

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Shakespeare has been allowed to hide his life for too long. Everyone praises him for his elusiveness, his gentle scepticism, his absence, his willingness to remain in uncertainties and doubts, his chameleon-like ability to sympathise equally with all his characters. But he didn't sympathise equally with one and all: he manifestly loved the Epicurean ways of Falstaff and Cleopatra more than the politic manoeuvring of King Henry IV and Octavius Caesar. It is time that we attached a philosophical stance to the mind of Shakespeare: Epicureanism. He was the greatest "militant idiot" of them all.