Appalling, twisted and dark: Ray Monk on Isaiah Berlin's attempt to revive the reputation of a philosopher neglected even by his admirers: The Magus of the North - Isaiah Berlin: Ed. Henry Hardy John Murray pounds 14.99

THINK less and live more. Such, in a nutshell, is the philosophy of Johann George Hamann, an 18th-century German thinker famous in his day as an opponent of the faith in reason that characterised the Enlightenment, but now read only by a few dedicated scholars. In the fervour of his anti-rationalism he was (to use a comparison repeatedly invoked by Berlin) a sort of D H Lawrence of his day - except that he did not write brilliant novels and short stories, but essays, reviews and fragments that, as Berlin is the first to admit, are all but unreadable. Berlin describes Hamann's writing style as 'appalling . . . twisted, dark, allusive, filled with digressions, untraceable references, private jokes, puns within puns and invented words'. When he goes on to say that Hamann's life, style, faith and thought 'were one', we begin to wonder exactly what he thinks is to be gained from studying Hamann's work.

The answer, it seems, is a better understanding of 'modern irrationalism', a tradition of thought that Berlin takes to include both 19th-century romanticism and 20th-century fascism. Hamann is important as 'the first out-and-out opponent of the French Enlightenment' and as 'the true founder of a polemical anti-rationalist tradition which in the course of time has done much, for good and (mostly) ill, to shape the thought and art and feeling of the West'. Berlin writes neither as a champion of Hamann's views nor as an admirer of his writing, but rather as a historian of ideas determined to apportion influence - and blame - where it is due.

Such a concern is entirely laudable, but it is not a standpoint from which an enticing or a satisfying book can be produced. Perhaps this is why, though most of the material which forms this book was written in the Sixties, it has remained unpublished until now. Indeed (so he informs us in his 'Editor's Preface'), until Henry Hardy came upon the collection of neglected drafts which make up this book, Berlin had forgotten he had ever written them: 'So thoroughly had Berlin dismissed this material from his mind that, until I found it, he assured me that nothing of the kind existed.' Hamann's fate, it seems, is to be forgotten, even by those who have set themselves the task of reviving interest in him.

The book that Hardy has constructed from Berlin's drafts follows the conventional form familiar to readers of, say, Oxford's 'Past Masters' series or Fontana's 'Modern Masters': a chapter on the life followed by chapters on the thought (in this case under the headings: The Enlightenment, Knowledge, Language, Creative Genius and Politics).

The biographical chapter is, alas, unrewarding and frustrating. Hamann led a fairly uneventful life: he was born in Konigsberg in 1730, attended the university there and, apart from a curious and unexplained visit to London in 1757, kept himself, his common-law wife and four children by a combination of a series of ill-paid posts with the Prussian civil service and some freelance literary work (the essays, reviews and fragments referred to earlier). He died in 1788, his reputation as the 'Magus of the North' assured by his having attracted as a disciple no less a person than Johann Gottfried Herder. The central event in his life - the key, one assumes, to understanding both his work and his personality - was a spiritual crisis and a subsequent religious conversion he underwent during his time in London. About this event, however, Berlin has disappointingly little to say.

Hamann's conversion shook him out of the conventional Enlightenment attitudes he had absorbed from his university education and inspired him to undertake a detailed and impassioned study of the Bible, the result of which was a fervid return to the Lutheran Protestantism of his childhood. From then on, his work took on a polemical - one might almost say a Messianic - tone. He was the prophet Elijah appointed by God to point out to the wicked king Ahab (Frederick the Great) the error of his ways. Like William Blake, Hamann was concerned to warn men of the folly of replacing the warm love of the human heart with the cold indifference of mechanical reason. Unceasingly, he set his face against the intellectual currents of his age and poured out a relentless series of polemics against, for example, Descartes' theory of knowledge, Kant's Critique of Pure Reason, the political and social ideals of the French philosophes, and anything else that elevated human reason above the passions and political progress above the salvation of individual human souls.

Berlin presents these views with sympathy and lucidity, but also with an understandable determination to distance himself from them. The largest claims he makes for Hamann as a philosopher centre on his philosophy of language, which (as Berlin demonstrates in an Appendix), when stripped of its theological underpinnings, looks something like Wittgenstein's. This is interesting, but a slender basis upon which to resurrect interest in the man. Berlin writes about him much as Bertrand Russell used to write about Lord Byron (or, indeed, about D H Lawrence): with a mixture of bemusement, fascination and, ultimately, disapproval that irrationalism could be so attractive.