Rep. Lee H. Hamilton (D-Ind.) is one of the remarkable members of Congress -- independent, thoughtful and in every respect reliable. That is not a fresh discovery. But it is a conclusion the country is coming to share as it watches him function as cochairman of the joint House-Senate hearings on the Iran-contra affair.

Those of us who were here 22 years ago took the measure of this tall Hoosier lawyer and Methodist minister's son when he became president of the big class of freshman House Democrats elected on Lyndon B. Johnson's coattails in 1964. As class president and as the man who had beaten a longtime Republican incumbent in his southern Indiana district, Hamilton was a White House favorite.

In his first term Johnson arranged three major public-works projects and 13 post offices for his district. But when it was time to blow the whistle on the frantic, incessant legislative demands Johnson was placing on Congress in his grandiose desire to build ''a Great Society,'' Hamilton did not flinch. In September of 1965, freshman Hamilton wrote a public letter to the president, calling for ''a pause'' for reflection and ''a more deliberate pace of legislation in 1966.''

Johnson was furious, but Hamilton was right. And his insistence on consultation -- aggravating to a series of successor presidents -- also has been correct. As former chairman of the House Intelligence Committee and ranking Democrat on Foreign Affairs, Hamilton is as consistent in his principles as he is faithful to his long-out-of-fashion crew cut.

His colleagues of both parties know this, and now the American public is finding it out. Hamilton's summary of the lessons of the hearings, delivered last week when Fawn Hall's testimony ended the first phase of the investigation, brought the confusing detail of 18 witnesses and 110 hours of testimony into clear perspective.

The principal lesson, Hamilton said, is that presidents get in hot water when their administrations abandon legitimate ways of operating. ''Our government cannot function cloaked in secrecy. It cannot function unless officials tell the truth. The Constitution only works when the . . . branches of government trust one another and cooperate.''

''Privatization of foreign policy is a prescription for confusion and failure,'' he went on. ''The use of private parties to carry out the high purposes of government makes us the subject of puzzlement and ridicule.''

Finally, he added, there must be accountability. ''High officials cannot look the other way or distance themselves from key aspects of policy or the actions of those they supervise. Accountability requires rigorous oversight by the Congress and a full exercise of the process of checks and balances under the Constitution. It requires above all the operation of the normal processes of government."

When I went to see Hamilton the other day, I asked the obvious question. Having watched presidents from Johnson to Reagan commit excesses of one kind or another, how do we ensure adherence to ''the normal processes of government?'' Hamilton's answer is that it is a question of attitude more than of law, of people rather than legal prescriptions.

''I don't think we will be making massive new legislative recommendations,'' he said. ''A lot of changes are already occurring as a result of the process'' of investigations by the Tower commission, the congressional panels and the special prosecutor. ''Don Regan is out, and Howard Baker is in. John Poindexter is out and Frank Carlucci is in. The president already has said he will no longer use the National Security Council for {secret} operations but will restore its advisory role.''

While Hamilton talked publicly in a weekend TV interview of the possibility of impeachment proceedings if evidence develops that Reagan knew of the diversion of funds to the contras, he does not want to put further restrictions on the authority of the president. He told me he believes strongly in the existing legal requirement that Congress be notified of covert operations. But even with all he has seen and heard of the Iran-contra affair, Hamilton said he would not join those Democrats who say, ''a president should not conduct a covert action without approval of Congress. I think a president has to have authority to conduct secret operations, so long as Congress is notified.''

But then he added an important proviso -- which somehow presidents have great difficulty learning: ''Secret operations should supplement existing policy, not take new initiatives,'' Hamilton said. ''In this case, they were at odds. The United States was urging all nations to refrain from selling arms to Iran or Iraq, but secretly we were selling arms to Iran. Our secret policy contradicted our public policy.''

That is what subverted ''the normal processes of government'' and the credibility of the Reagan administration, with costs being felt everywhere from the Persian Gulf to the 1988 presidential preliminaries. If Hamilton is right again in judging that Congress will not find legislative remedies for such folly, we have to be sure those seeking to succeed Reagan understand the fundamental lesson of this sad affair.