One of the earmarks of the flexibility in the American system of government is its ability, at certain times, to deal with public questions in a logical, deliberative fashion and, at other times, to postpone those questions until what appear to be urgently needed actions are taken.

When President Truman proposed the Marshall Plan and the NATO treaty, Congress paused to consider and debate the implications of a permanent American commitment to the military security and economic prosperity of Western Europe. But when Presidents Roosevelt and Johnson were shoving through the measures that came to be called the New Deal and the Great Society, they did ask Congress or the country to stop and weigh the overall advantages and risks of sharply expanding public-sector expenditures and the scale of government.

Ronald Reagan is operating very much in the Roosevelt-Johnson style. He is pressing for action and postponing debate. No matter that inflation seems to be abating and the economy is rolling with unexpected vigor; Reagan insists that his budget and tax cuts are needed to deal with "the worst economic crisis" since the Great Depression.

No matter that the Soviet Union is hobbled by shortfalls in its agriculture and industry, is bogged down in Afghanistan and baffled by the Solidarity movement in Poland; Reagan asserts that record peacetime increases in defense spending are needed to cope with the Soviet threat.

I do not criticize Reagan for this. What he is doing is what strong and self-condident presidents before him have done. He is capitalizing on the momentum of his election victory, the disarray of the political opposition and the public support for his leadership in order to push through as much of his program as possible before the inevitable second thoughts about the wisdom of his policies occur. Like Roosevelt and Johnson before him, he is seizing the moment -- knowing that the question is not if, but when, his leadership will be challenged.

The American system permits such efforts to succeed but rarely, and Reagan is wise enough to recognize he has such an opportunity.

But on this long holiday weekend, with things crawling to at least a temporary halt in Washington, it is possible to step back fro the frenetic pace of executive and congressional action of the past four months, and note some of the large, unexamined propositions underlying Reagan's program.

When I say "unexamined," I do not mean that Reagan himself or his aides are unaware of where they are going. Quite the contrary. The blueprint is exceptionally clear to those in control.

But the propositions are unexamined in serious political debate. Jimmy Carter's infirmities impeded such discussion in the course of the fall campaign, and no critic has had the platform from which to challenge the Reagan policies since Election Day.

But do not doubt that such a test is coming. It is guaranteed by the very sweep and boldness of the policies Reagan is rushing through. Consider some of the propositions implicit or explicit in the Reagan program, and ask yourself if any or all of them can long escape serious, skeptical examination. To support the Reagan program, you must believe with him that:

Almost every disruptive and disturbing development in the world reflects Soviet scheming or power-wielding: the massive expansion of America's counter-threat is the only effective way to stabilize the international scene.

Federal taxes and regulations are the main barriers to economic growth, and federal spending is the main cause of inflation: a radical reduction of the federal role in the economy is the only way to engergize the economy and stabilize its growth.

There is a natural harmony between the interests and inclinations of business managers and their employees, customers and neighbors: freeing the owners from government restraints will automatically work to the benefit of everyone who deals with them.

State and local governments are more efficient and equitable in their distribution of public funds and services than the national government: therefore, turning program responsibility back to them will both save money and increase public satisfaction.

In this new environment, individuals, families and private organizations can be relied on to replace government in a wide variety of roles, ranging from support of the arts and scholarship to the financing of retirement: social needs, and not just private consumption desires, will be best satisfied by a major shift of resources to private hands. d

These are just a few of Reagan's major propositions. Everything in our history suggests that, sooner or later, they will be tested. Questions like these can be postponed, but they cannot be safely ignored.