By now most people with an interest in art must be aware that a huge and superb Manet retrospective cosponsored by the Louvre and the Metropolitan Museum has just opened at the latter museum here. And that critics are already describing it as an ''eye-opener,'' as affording a ''new insight'' into the nature of Manet's genius.
If that is true, at least as far as the critics are concerned, I cannot help wondering just how carefully they have looked at the many Manets in American collections, or for that matter, the Manets reproduced so beautifully in so many books. There is no question that this is a truly magnificent exhibition, and probably the best Manet show we will see for decades to come. But to stress how revealing it is of Manet's true qualities is to admit to having taken for granted (or to having not really studied) the superb Manets of all periods owned by the Metropolitan, the National Gallery of Art, and various other American museums.
Manet, I'm afraid, is one of those artists to whom extensive lip service but very little real attention is paid. Every-one knows he was a great painter and that he scandalized Paris in 1863 with his painting ''Le dejeuner sur l'herbe,'' a large work that depicted two fully dressed men and two unclothed women picnicking in a park. But he has never quite caught on with the public the way Monet, Renoir, Degas, Van Gogh, or Lautrec has.
But then, that's not surprising. As a painter, Manet is a bit too cool, quiet , and detached, and too indifferent to painterly flair, ever to be truly popular with the public - or even with many art professionals. Art historians certainly give him very high marks, but he actually belongs more to his fellow painters, and to all serious students of what it means to paint, to build form through pigment, or to create harmonies through subtle adjustments of values and hues. He was, first and foremost, a problem-solving painter whose genius and art-historical importance lay in his ability to set and to resolve remarkably relevant and challenging painterly problems. To make anything else out of him, to stress his relevance to 20th-century sensibilities and realities on the basis of his work's ''modern'' look or the directness of its execution, is to put the emphasis on his art's appearance rather than on its substance.
Manet is a painter's painter - and one of the very finest at that. There was not much he couldn't do with paint. For many years I couldn't pass through the large room at the Metropolitan previously given over to his work without stopping in near-disbelief at the silver and cream tonalities in ''Woman With a Parrot,'' the extraordinary reduction of character and form to a few shapes and tones in ''The Spanish Singer,'' or the literally dozens of other examples on view of painting at its purest and best.
No, the depth and range, the quality of Manet's artistic greatness, have always been very much in evidence here. It should not have required a huge and major retrospective of 190 of his works to clarify it, or to provide fresh insights into the nature of his genius.
On the other hand, this exhibition is a perfect opportunity for everyone to see old favorites and hitherto unknown works brought together for the first time , to discover a few new facts and details about Manet and his work, and simply to enjoy the art of one of the best painters of the 19th century.
It should also serve as an excellent introduction to Manet's work for those not yet familiar with it, and as a veritable treasure-trove for younger painters eager to learn all they can about paint. It is huge, detailed, and thorough, and lacks only a handful of his masterpieces not lent to the Metropolitan for the occasion to make it really complete.
Their absence does not, however, weaken the show in any serious way. The 95 paintings, 45 drawings, watercolors, and pastels, and 50 prints that are included do full justice to Manet's art. These range in date from 1859, when Manet was 27, to 1882, the year before his passing, and cover every aspect of his career.
And what a career it was! That one man could paint so much so well in only a little over 20 years is difficult to grasp. There seems to have been very little floundering and confusion on his part, even as a very young man. And once he got started, his growth on every level was truly astonishing.
I cannot recommend this exhibition highly enough. It's a major art-world event and a first-rate viewing experience. It will remain open to the public through Nov. 27, and then will be disassembled.
For those unable to make it to New York to see the show, I heartily recommend the exhibition catalog. ''Manet'' is a 548-page gem of a book with superb color and black-and-white illustrations and an excellent text. It was published by the Metropolitan Museum, and sells for $39.50 (hard cover) and $25 (soft cover).