William Proxmire of Wisconsin is one of the most valuable members of the U.S. Senate. G. William Miller, the businessman confirmed Friday to be head of the Federal Reserve Board, is - to me, anyway - an unknown quantity.

But I found myself exultant when Miller stood up to Chairman Proxmire during his confirmation hearings by the Banking Committee. For Miller was in the rare position of fighting the maverick irresponsibility that is the beseting sin of American public life.

Proxmire needs no endorsement from me. So acute a judge as Arthur Burns, the outgoing chairman of the Fed, has called him "one of the most intelligent men in the Senate."

He is honest, hard-working and blessed with the rare gift of attracting strong staff. By his "Golden Fleece" awards he has played a notable role in making liberal Democrats more sensitive to waste in government. Unlike almost all his fellow senators, he has been conscientious about confirmation hearings, and he earned the prophet's mantle by alone opposing the nomination of Bert Lance as director of the Office of management and Budget.

While fine for the odd individual, however, Proxmire's populist independence has its limitations when it comes to running things. Society is not, and cannot be, simon-pure. The leader of any group endeavor has to make allowance for human frailty.

Proxmire departed from that norm in conducting the hearings on Miller's nomination for the Federal Reserve. During the initial hearings in January, Proxmire developed some evidence that the Carter administration had completely overlooked in making the appointment.

Bell Helicopter, a company owned by Textron, the conglomerate headed by Miller, had sold $500 million worth of helicopter to Iran back in 1973. It had paid a fee of $2.9 million to the Iranian agent for the sale - a company called Air Taxi.

Proxmire, with some help from the CIA, made it pretty clear that Air Taxi had included among its silent partners Gen. Mohammed Khatemi, chief of the Iranian Air Force and brother-in-law of the shah. The inference was that Textron had bribed the royal family to sell helicopters.

Miller, in response to questions, expressed doubt about Khatemi's role in Air Taxi. He asserted that, in any case, he had not personal knowledge of the transaction.

The hearings were adjourned for several weeks of investigation. The investigation showed that Textron - unlike several other firms - had conducted only a limited audit of its military sales, and had not availed itself of a chance to be absolutely cleansed through an investigation by the Securities and Exchange Commission. An SEC investigation is now under way. But nothing conclusive, or even suspicious, was proved against Miller personally.

Despite the absence of any proof, Proxmire - citing the SEC investigation of Textron and the possibility of a court case - suggested that Miller had a "responsibility" in the "best interest of the country," to step down.

Miller gave him the retort neat. He said: "My withdrawal from this nomination would be the most irresponsible thing I could possibly do. It would be irresponsible to this committee . . . irresponsible from the point of view of the president, irresponsible from the point of view of the nation."

The committee was obviously impressed. It confirmed Miller, with only Proxmire voting against him.

I have no idea whether the SEC investigation will clear Miller or Textron. Still I applaud his words. For they pose a sharp question to what has become the dominant attitude in American public life since Vietnam and Watergate. That is the attitude that anything that has enjoyed a certain success over the years has to be part of an awful thing called the establishment; that behind every establishment there is a secret; and that behind every secret there is a dirty secret.

That nation is widespread in the press and television, notably among investigators and showbiz personalities. It defines the central animus of the reformers who control Congress. It is dear to the heart of the Carter administration, and indeed describes the platform on which Carter was elected.

But it is false to the core. It is a recipe for the unraveling of institutions and authority, particularly the authority of government. So Miller's rare challenge to that populist doctrine merits at the very least two cheers.