THERE are many ways of characterizing the polymathic Isaiah Berlin. One that springs handily to mind is "liberal humanist."As even a quick dip into his writings makes clear, however, being genuinely humanistic and truly liberal is not an ideology, but a hard-won achievement based on cultivating the capacity to weigh conflicting ideas and values. Berlin is a champion of pluralism who nonetheless rejects the easy and demoralizing notion that "everything is relative." Born in Latvia in 1909, Berlin was educated in England, where he went on to pursue a distinguished academic career as a philosopher and historian of ideas. During World War II, he worked at the British embassies in Washington and Moscow. Throughout his long career, Berlin's influence has been felt as much outside the walls of academe as within. If not quite a popularizer of ideas, he has done much to disseminate them and to shape public discourse. Most of the 15 essays in "Isaiah Berlin: A Celebration" were originally intended to be read at a conference in Jerusalem honoring Berlin on his 80th birthday in 1989. The conference did not take place, but the essays were written. Some, like Joseph Brodsky's tribute, are touchingly personal. Others, like G. A. Cohen's "Isaiah's Marx, and Mine," mingle personal reminiscences with a more theoretical look at Berlin's ideas. The essayists pay tribute to Berlin by writing about topics that have long been of special interest to him. A brief summary of the contents demonstrates the remarkable range of his interests: Stuart Hampshire examines Berlin's concept of nationalism. Michael Ignatieff questions how well Berlin - or anyone else - h as managed to account for a phenomenon that is related to, yet distinct from, nationalism: fascism. Yael Tamir applies Berlin's thoughts on the importance of ethnic identity to recent movements like feminism. Leon Wieseltier and Ronald Dworkin offer investigations of the consequences of Berlin's famous distinction between "negative liberty" (which Berlin defines as the absence of legal restraints on personal freedom) and "positive liberty" (a more problematic concept involving the individual's ability to achieve self-realization). Wieseltier applies this distinction to the legal issue of separation of church and state; Dworkin tackles the issue of freedom of speech versus the harm that women may suffer from pornography. Further reflecting the diversity of Berlin's enthusiasms, pianist Alfred Brendel offers an essay on music and humor, Francis Haskell one on Italian art, Bernard Williams one on opera, and Stephen Spender a poem. The volume is a fitting tribute to a thinker famed for his erudition, eclecticism, and clarity of style. Berlin's own writing is an even greater delight. His lucid, colorful essays, scattered among a variety of journals, are being collected into books. The most recent, "The Crooked Timber of Humanity," is the fifth in a series being edited by Henry Hardy. The first volume, "Russian Thinkers," includes Berlin's famous essay on Tolstoy, "The Hedgehog and the Fox.Concepts and Categories," the second, discusses issues in philosophy. "Against the Current" focuses on thinkers who questioned the rationalistic assu mptions of the Enlightenment. "Personal Impressions" are portraits of influential contemporaries - Churchill, Roosevelt, Einstein, Chaim Weizmann, Aldous Huxley, Anna Akhmatova, and others. (These four books, first published by Hogarth in Britain and Viking in the US, have just been brought out in paperback by Penguin.) The title of "The Crooked Timber of Humanity" comes from a remark of Kant's, loosely translated, "Out of the crooked timber of humanity no straight thing was ever made." The eight essays reflect Berlin's conviction that "scientific" accounts of human nature - efforts to predict human history or devise social utopias - are doomed to fail because they do not take account of the elements of human nature that are quirky, individualistic, irrational, and resistant to generalization. Most of these essays written in the 1970s and 1980s deal with the theme announced in the opening piece, "The Pursuit of the Ideal," which was originally delivered (in a shorter version) on Berlin's receipt of the Agnelli International Prize (for the "ethical dimension in advanced societies") in 1988. "The search for perfection," he warns, can be a recipe for bloodshed." Only by recognizing that moral values can collide and learning to tolerate differences can we avoid the human suffering caused by rigid ideologies. The conflict between the universalism of the French philosophes and the nationalism of the German romantics forms the background to Berlin's treatment of 20th-century politics. While he concedes that technology and ideology have been the forces wreaking the greatest change - and havoc - in our century, he reminds us of the importance of another 19th-century force whose consequences were far greater than foreseen: nationalism. Nationalism began as a defense of local customs but mushroomed into something more ominous. For all that Berlin detests national chauvinism, he tends to gloss over the downside of nationalism in his eagerness to trace it to its liberal roots: the desire for self-determination and the belief that national and individual idiosyncrasies should not be sacrificed on the altar of "universal" social engineering. Although he is aware of the dangerous point at which pride in one's nation becomes contempt for othe r nationalities, he pays relatively scant attention to the ways in which national, tribal, or religious loyalties can stifle the freedom of individuals. The longest essay, published here for the first time, is a brilliant reassessment of the pessimistic counterrevolutionary thinker Joseph de Maistre. A foe of the Enlightenment who admired the absolutist rule of Louis XIV, de Maistre saw mankind as weak, sinful, redeemable only by obedience to authority, and driven by a need for punishment and self-immolation on the altar of some mysterious "higher cause." This kind of thinking, Berlin points out, seemed outmoded to de Maistre's 19th-century contemporarie s, but proved horrifically prophetic of the rise of fascism in our own time. Each essay in this collection provides an elegant encapsulation of a central theme in Berlin's thought. The cumulative effect of reading them is to increase one's admiration for their coherence and common sense. But the reader also gets to see - and regret - some of his blind spots as well.