The Poetick Fields that appear versions of God's own Paradise

Scholarly, irreverent, personally involved: the three rarely seem to go together, but those are the qualities that drive this innovative new study of an old subject. Ever since Nikolaus Pevsner decreed that the only original English contribution to European art was the 18th-century landscape garden - Stourhead, Stowe, Castle Howard - the experts have been puzzling over its origins. Tim Richardson has concentrated on the personalities involved in creating these Arcadias and the connections between like-minded friends: links between Sir William Temple, William Bentinck, Constantijn Huygens, the poet and diplomat Matthew Prior, Gaspar Fagel, Grand Pensionary of the Netherlands, and Henry Compton, Bishop of London - all keen gardeners.

There are disturbing results. Richardson's central argument is that a sharply geometric garden produces at eye level a pleasant chaos and intimacy of parts, the "varieties" advocated by Alexander Pope and Joseph Addison. So Richardson believes that the "English Garden", which took the Continent by storm, was really Dutch in inspiration. Or, even more surprising (and if Bentinck's visit to the serpentine Bosquet de Louveciennes at Marly is considered), derived from the French.

In 26 chapters with wittily provocative titles such as "Agents orange", "Pope pops up" and "Sex, death and grotwork", Richardson explores the idea and its protagonists. His pen portraits of eccentrics such as William Shenstone, the poet gardener of Halesowen, are richer and deeper than any earlier accounts. He has read all the secondary, and many of the primary, sources and develops them rewardingly. One tends to take Addison's successful play Cato for granted, but not Richardson. One of Cato's sons, Portius, has read Paradise Lost and picked up the key garden idea that Heaven has informal complexities. So already, in 1713, Addison was projecting serpentine woodland walks as versions of God's own Paradise, destroying the Platonic ideal of unity underpinning formal gardens. "Uniformity amidst Variety", as Francis Hutcheson was to put it later in 1725, could lead to greater aesthetic stimulation.

Richardson redelivers the past in present language; his text is unashamedly populist, but his material is consciously erudite. That golden haze on the average Claude Lorraine painting is "like an all-over fake tan"; Locke was the "Che Guevara of the radical Whig cause. Without the machine gun. Wearing a Whig". I winced briefly on reading that Theocles, the wise old philosopher in Lord Shaftesbury's Characteristicks of Men, Manners, Opinions, Times was "presumably having a lie-down and a cigarette" while Philocles came back with a reasoned response to "bodice-ripping metaphysics". But do we distance ourselves by deference from the Georgians? Was Addison the Peter Mandelson of his time, robbing the Tories of a populist philosophy with his aesthetic and economic deism, leaving them in the political wilderness for the next 40 years?

Richardson's most unnerving comparison does explain Locke's theory that our perception of the world is as important as its realities: a young man on the Grand Tour, or in Richardson's terms, "a sex-crazed adolescent", would see Rome, after absorbing the classics endlessly at school, in the same way as we see New York, landing there after a diet of Superman and Kojak . As Addison wrote: "Poetick Fields encompass me around,/ And still I seem to tread on Classick Ground". It accounts for that urge to scatter a landscape park with classical temples.

Stephen Switzer, who had worked for the gardeners George London and Henry Wise and knew what he was talking about, named the Earl of Carlisle's Wray Wood at Castle Howard as one of the most influential early attempts at an Arcadian garden. But Richardson, who walks his parks and gardens as thoroughly as he reads his sources, found all Wray's "varieties" long gone. He might have ducked the flying golfballs at Heythrop in Oxfordshire; it was there that Switzer claimed, as early as 1710, that he had seen with emotional delight "the first attempt of this kind"; and Heythrop's Arcadian Grove, with its Cold Bath and Nymphaeum, built for the first Duke of Shrewsbury, is intact, forlorn and atmospheric. If there is one distinct fountainhead of the 18th-century Arcadian garden (and after visiting 560 gardens in the past seven years, I have my doubts), then it is waiting to be hailed as such at Heythrop.

Timothy Mowl is professor of history of architecture and designed landscapes, Bristol University.