PAUL MELLON'S GIFT to Yale of its new Center for British Art is an exemplary personal achievement as well as an extraordinary act of philanthropy. It was Mr. Mellon who conceived the idea of enriching his alma mater in this fashion. It was he who paid $12.5 million for the beautiful building, the last major work of the late Louis Kahn. And it was Mr. Mellon - with the unsalaried aid of British art historian Basil Taylor - who put together the 1,800 paintings, 5,000 prints, 7,000 drawings, 16,000 rare books, and so on. Our art critic, Paul Richard, has called the Center "the finest collection of British art ever privately assembled." Combined with Yale's Boswell and Walpole papers, the university becomes perhaps the leading center of British scholarship outside Britannia herself. And the bequest, of course, is but one of many Mr. Mellon has made to Yale and to others - particularly in the Washington area, where he lives.

What seems even more noteworthy, though, than the scale of Mr. Mellon's giving in his way of giving. The Yale Center, in particular, exemplifies Mr. Mellon's quiet, intensely committed kind of philanthropy. British art is not a field in which to invest if one is out to make a publci splash (and with Mr. Mellon's means, such splashes could hardly be easier, if he so chose). But with Mr. Mellon, collecting is inextricably a part of his genteel, cultivated style of living. To go out and scour the world for opulent artifacts that would not fit comfortably with that way of living would smack of tasteless and undisciplined acquisitiveness. The British bequest is rooted in his attachment to the British country landscape, to the sporting life and to British learning. Thus he made himself a champion of those gentle English landscapes, stately portraits and pristine sporting scenes that were, as Mr. Mellon has noted, "long neglected, even abandoned, not only in this country, but in their homeland."

A not unrelated characteristic of Mr. Mellon's philanthropy is exemplified by his determination that the project be called the Yale Center, not the Mellon Center - just as his father, Andrew Mellon, declined to have the National Gallery of Art named after him, though he built it and donated his resplendent collection of Old Masters. The same will be true for the additional National Gallery structure now being built by Paul Mellon - to more than double the gallery's space - even though it is very much his building. He chose the architect, I.M. Pei. Mellon money (one-third of it Mr. Mellon's own) will pay the bill of about $100 million. And as the anything-but-absentee president of the gallery (his "Who's Who" listing is "art gallery executive"), he supervises construction as if it were to be his residence. But a hard hat labeled "Mr. Mellon" that hangs in the construction shed is about as close as he will allow his name to be attached.

In a speech some time ago, Mr. Mellon voiced his view on philanthropy, observing that "considering the blessings of a large personal income, added to the favorable climate brought on by enlightened and liberal tax regulation, one is forced to admit that the doing of good deeds was wished onto one by a sort of happy predestination." The result is a "happy predestination" for more than just Mr. Mellon.