WHEN WE THINK of Bernard Berenson, the image that comes to mind is of a white-bearded sage, surrounded by books and beautiful paintings in his Villa I Tatti near Florence. The utter perfection of the image has often beguiled memoirists who knew him in his later years into writing about him as if he were a monument to be venerated rather than as a man to be understood.

This worshipful attitude even shines through the sophistication of the show called "Berenson and the Connoisseurship of Italian Painting," on view at the National Gallery of Art until Sept. 3. Thus the curator who organized the exhibition, David Alan Brown, has not found it possible to think of Berenson as having grown up in an era of entrepreneurial lawlessness. As a result, in the handbook which accompanies the show, Brown takes up the issue of Berenson's unethical conduct as a connoisseur only in order to dispose of it." "Berenson," we are told, "unquestionably overpraised pictures in whose [sic] acquisition he had a stake." Yet no sooner is this admission made than the handbook assures us that "there is no evidence to suggest that he evermade an attribution he did not genuinely believe at the time," and then rounds off the whitewash with an anecdote in which Berenson's "integrity as an independent scholar" is contrasted with the hard-sell hucksterism of the art firm headed by Sir Joseph Duveens, the flamboyant dealer with whom Berenson was associated for 30 years.

The aging Berenson himself, however, in Sketch for a Self-Portrait , admitted with considerable anguish that in the course of his career he had more than once betrayed his ideals. Although he did not enumerate them, those betrayals included - in addition to making absurdly inflated statements about paintings he wanted his clients to buy - smuggling Renaissance treasures out of Italy in defiance of national law; putting clients in touch with people who knew how to evade U.S. customes duties on imported works of art; concealing from clients his financial interest in art objects on which he prented to be offering distinterested opinions; and resorting to elaborate strategies of dissmulation when his principal client, Isabella Stewart Cardner, apparently grew suspicious that he was accepting kickbacks from dealers.

Fortunately, Ernest Samuels, professor of English, emeritus, at Northwestern University, has finally broken through the pieties of the Berenson cult. In a meticulously researched and attractively written account of the first four decades of the connoisseur's life - a second volume, dealing with the later career, will be published sometime during the 1980s - he shows us a young man of deeply divided identity who was simultaneously driven by the visionary aestheticism of Walter Pater's Marius the Epicurean and the will to power of a Nietzschean superman. The recuring bouts of nervous illness to which Berenson was subject in his 20s and 30s were an indication of the psychic price he paid for his inability to resolve the towering contradictions in his personality.

He was born Bernhard Valvrojenski, to Jewish parents in a muddy village in Lithuania. In 1875, when Bernhard was 10 years old, his family immigrated to Boston and took the name of Berenson. (Not until the Uited States entered World War I, however, would Bernhard become Bernard.) The boys father, an unsuccessful peddler of cotton cloth and laces, was able to keep his intellectually precocious son out of the work force and send him to the Boston Latin School, but could not possibly have paid for his education at Harvard. Exactly who provided the wherewithal for that experience is unknown, but it is reasonable to assume that various members of the city's social and intellectual elite had already taken an interest in him, even as a group headed by Mrs. Gardner would finance a year abroad for him after his graduation.

If proper Boston's largesse was tinged in certain instances with anti-Semitic condescension, it was a prejudice that the young Berenson was only too eager to ape. For years, his correspondece recorded his disdain for "Jews and other indecencies." While at Harvard, moreover, he was baptized as an Episcopalian, and some years later in Europe he formally embraced Roman Catholicism. That neither conversion lasted very long is a further illustration of his fundamental uncertainty about who he really was.

Yet his psychological problems never disabled his formidable inteligence; indeed, they may have helped him to mobilize it. For Berenson's efforts to identify himself mirrored his greatest achievement as an art critic - the attribution of authorship to works of art, and the authentication of those works as either originals or copies.While he also gained fame as an art theorist, in four early books on the Italian painters of the Renaissance, he was more adept at coining catchy phrases - "tactile values" was the most famous of them - than in formulating powerful and enduring concepts. The lists accompanying those books, however, in which Berenson tallied the pictures he accepted as authentic, have had a lasting influence on the thinking of art historians, and so has the great catalogue of Florentine drawings which he published in 1903.

On the occasion of the 50th reunion of the Harvard class of 1887, Berenson reported to his classmates that most of the Italian paintings which had come to the United States had "my visa on their passport." Enormously wealthy American collectors had placed their faith in Berenson's expertise because their own taste was shifting - for reasons not adequately explained by Professor Samuels or by anyone else - to the art of the Renaissance, and they wanted the best advice they could get. That the United States now possesses the most distinguished collections of Italian Renaissance paintings outside of Italy is a tribute to that historic shift, and to the art connoisseur who understood it as the opportunity of a lifetime. CAPTION: Picture, Berenson at the Villa I Tatti in 1903