Are we spread too thin, Mr. President? No, we are not, the president said to this question on Tuesday, having just reaffirmed his Lebanon deployment and put some 1,900 American servicemen ashore in Grenada.

It is a slippery concept, being "spread too thin." As the fellow said who was asked if he loved his wife, compared to what? In his denial Tuesday, Reagan seemed mostly to be trying to fend off the implication that he had imprudently gotten the United States in over its head.

The political fact is that many people believe President Reagan has spread us too thin.

Conservatives, including many in the Pentagon, believe it. For military people, who are in the business of anticipating contingencies, it did not take these latest events to make them think that the strain on American military resources is great and growing. They conclude that we will need even more men, ships and aircraft than the president's immense rearmament campaign provides.

Liberals, from their own perspective, also believe we are overextended. They mean we are amassing and using force carelessly. The latest events they see not as justification for more hardware and bigger defense budgets but for more presidential discretion and more respect for complexity and diversity in international life.

Consider the administration's circumstances at this moment. Reagan is conceivably not all that far away from intervening in some way in Nicaragua, which has the means and no doubt the will--look how the Cubans fought in Grenada--to put up a formidable resistance. He is being advised by some quarters to react to the Beirut disaster by bombing the Syrians (and any Soviet companions) in Lebanon's Bekaa valley. There is talk of going after the Iranians, either for what they may have done in the Beirut bombing or if they move their war with Iran into the stage of sealing the Persian Gulf.

One can understand what must be the immense frustration of Reagan and others who support the scale and rationale of his defense buildup to find even little flyspecks like Grenada acting as though they did not have to bend their policies to American power. There is the added frustration of suffering an agony and humiliation in Lebanon without even having the consolation of knowing for sure on whom to unleash the tremendous American power available for vengeance.

But if the president did just one or two things off the list of current extra military possibilities, on top of Lebanon and Grenada and what goes on elsewhere as a matter of military routine, we would be spread "too thin" by almost anyone's definition.

There is a further problem. At the core of Reagan's thinking, and not only his, is the idea that the stress and unrest in the world constitute an objective reality, something that exists independently and that is dangerous to the United States and that poses to us the unavoidable question of what to do about it.

What is missing from this idea, among other things, is the companion idea that foreign-policy requirements exist not only absolutely but relatively--threats must be perceived and measured against the resources on hand to meet them. This can mean the behavior of an ostrich, but it need not. It should mean prudence or, in Walter Lippman's famous phrase, solvency: seeing straight but not taking on more than one can handle.

Some of us have leaned toward the Lebanon engagement and away from the Grenada one. But my point is that nowhere is it written that either or both had to done. Some part of what the country does lies within the irreducible ambit of deeds that any president would probably feel compelled to undertake. Some other part of a president's service of the United States' security, interests and commitments-- elastic notions all--lies within the realm of his personal judgment.

I wish I were more confident that the president and his advisers, official and unofficial, were looking at these possibilities in the round and doing some anticipatory arithmetic on their potential total costs and risks. At the least there is a need not to make statements or to allow plans to go forward that will make it more difficult to throttle back later if a no-go decision in any one case is finally made.

It is the difference between a reflexive president, who can keep us constantly stretched too thin no matter how fast and furiously we rearm, and a reflective president, who will slow down and try to ensure that the question does not arise.