MOST AMERICANS are dismayed by the system of justice in the United States, believing that it favors the rich instead of treating all people as equally as possible. Most think the income tax system is a disaster. Of those who have an opinion on the subject, most think President Reagan wants to invade Nicaragua, despite his statements to the contrary.

These are all findings from a recent Washington Post-ABC News poll, conducted in June. They paint a gloomy picture, one of public dissatisfaction and distrust -- not at all the mood portrayed by the many national leaders and boosters who say that "America is back."

But that Post-ABC News poll and others also show the public giving comparatively high approval ratings to Reagan. They often find a majority saying that America is moving in the right direction. And recent surveys have even found majority approval for the way the long-maligned Congress does its work.

Such findings, of course, paint a very different picture.

These apparent contradictions in polls have been giving opinion analysts fits in recent years. It is hard to explain how people can dislike Reagan's policies and distrust what he says, yet give him high ratings. It is equally baffling when majorities take the most pessimistic view of public affairs, yet say that things are going in the right direction.

Many analysts have "solved" the Reagan incongruity by calling him a Teflon president -- one who is so charming that people do not blame him for any problems they have with his policies. That view is probably generally accepted by now. The problem with it, aside from sounding kooky, is that it is wrong.

There is no Teflon attached to Reagan. During the recession of 1982, he got poor approval ratings. Many people gave up on his economic program of tax cuts and military spending increases, and with public opinion swinging against him, Reagan changed course and supported a $100 billion tax increase, calling it tax reform.

Later, public opinion cried for removal of U.S. Marines from Lebanon. Reagan said he would never "cut and run;" then, a few days later, he removed the Marines. Under pressure from public opinion, he also has backed down on military spending, Social Security cuts, and, one guesses, on the nature of the actions he takes in Central America.

There was no Teflon in those moves -- alert politics perhaps, but no Teflon.

Analysts generally try to explain this situation by either ignoring half the findings or by shrugging and saying, "There are always contradictions in public opinion polls" -- as though there is something wrong with the public, not with the experts.

It is time we had a better understanding of these apparent contradictions. They have been going on too long to think of them as a fluke. I think there is a simple explanation, one we might call the "porcupine theory."

The dismay the polls have picked up is real. America has not rebounded only a little from Vietnam, Watergate and from the jolts of the assassinations of John and Robert Kennedy and Martin Luther King Jr. Those events have led to extreme disenchantment and lowered expectations or demands.

Basically, people have turned their back on government. That helps explain why, despite a public that is much better educated than at any time in history, voter turnout remains in such a trough.

The overriding requirements of most citizens are that government keep the economy in some sort of order, so that the citizens can get by, and that it keep the country out of needless military conflicts, so that citizens are not in jeopardy.

The porcupine is not very aggressive. It extends its spines when aroused or attacked. The rest of the time the spines just lie there, and the porcupine lets the world go by.

Most citizens have always taken the porcupine's approach to public affairs, paying little attention except when events become personally touching. Economic hardship touches people personally. So does any threat of war.

When the economy is working, or seems to be working, and there is no immediate military concern, most citizens will give the nation's leaders decent or high grades and say that things are moving in the right direction.

Other issues and principles -- even deeply rooted ones such as equal justice under the law -- are secondary for most of those who are not directly touched by them. A sense of wrongful justice creates anguish with the system but not as much grief as an inability to make a mortgage payment or buy a birthday present.

Under the porcupine theory, national leaders have a great deal of latitude. So long as they keep the economy going well, or give the appearance of keeping the economy going well, they can do a good deal of what they want in other areas.

The Reagan administration, for example, is doing what it can to dismantle social programs despite opposition from a majority of the public.

The June survey showed 59 percent opposed to any substantial cuts in social programs, with only 37 percent saying the government spends too much tax money on social programs. The figures have been pretty much the same for the past 3 1/2 years. But in all probability, further cuts can be made with minimal public outcry. Not enough people see themselves as being touched directly.

Similarly, polls consistently show majority support for programs to clean up the environment, for education, for health care and the like, including programs with a decidedly liberal bent. It is said that the country has become more conservative and frowns on such programs, but the opinion polls show just the opposite when it comes to citizen priorities, presenting another apparent contradiction.

Reagan says it is not the role of government to solve most of society's problems. So long as the economy seems to be working well, people will put their doubts about that to the side and give him high grades.

But the next president could take exactly the opposite approach and also get high grades-depending on the economy. The ordinary citizen can go either way.