As long as Ronald Reagan has been in public life, he has been saying things that have made it hard for some people to take him seriously. The skepticism, it is clear, has not been an insurmountable barrier to his success; he is, after all, in the White House, and they are not.

He has never been accused of guile. Love him or not, people recognize that Reagan says what he believes. But even with the vast machinery of the government at his service, this president goes right on saying things that make you wonder where in the world he does pick up his ideas.

In a meeting with out-of-town reporters and editors 10 days ago, the president made some rather remarkable assertions.

The Russians, he said, "cannot vastly increase their military productivity because they've already got their people on a starvation diet of sawdust. . . ."

The demonstrations against U.S. nuclear weapons that had been popping up in Western European capitals were of no consequence, he said. They resulted from "propaganda that . . . can be traced back to the Soviet Union," but the protests were insignificant because none of the European governments was "falling back or falling away from the installation of these weapons."

At home, he said, the anti-inflation program "is going to be successful. . . . Our Nobel Prize winner, Milton Friedman, has just been quoted as saying it (inflation) will be down to 6 percent next year."

Well, Friedman is a respectable authority, but a few days after the president cited his wisdom, the consumer price index jumped back up to an annual rate of about 14 percent--recalling, to some people, that the last government that had embraced Friedman as an authority was that of the embattled British Tory prime minister, Margaret Thatcher.

The president did not name his source on the Soviets' sawdust diet, nor did he explain why, if the Russians are that close to starvation, we are bailing them out with our grain sales.

As for the contention that the anti- nuclear weapon demonstrations in Europe are insignificant, events have not been kind to the president's judgment.

Two days after the president met with the correspondents, the people of Greece--who are inured to communist propaganda--elected a new government pledged to the immediate expulsion of American nuclear weapons and eventual withdrawal from the NATO military alliance. A few days later, the NATO defense ministers insisted-- over American objections--on considering an offer to the Russians: to freeze any fresh nuclear-arms deployment in Western Europe if the Soviets would reduce their own nuclear missile fleet. Last weekend there were large anti-nuclear demonstrations in London and Rome that confirmed the warnings of the earlier demonstrations in Berlin, Brussels and Bonn.

Occasionally, it appears, President Reagan hears something from one of his unnamed authorities that causes him to revise his opinion. "I had always believed . . . the Voting Rights Act should have been nationwide, rather than taking out certain areas" for its targets, Reagan told his visitors, neglecting to mention that the act was applied only to areas with clear historical records of voting discrimination. "But then, I must say, it was brought to my attention recently that . . . it would make it so cumbersome--and I hadn't thought about this--that it might be impossible to enforce. So we dropped that position."

Before rejoicing at this triumph of pedagogy, note that in the very next sentence the president said that the Voting Rights Act extension, recently passed by the House of Representatives, is "maybe . . . pretty extreme." He hoped, he said, the Senate would pass a "more reasonable" bill.

That "pretty extreme" bill was passed 389-24, without a murmur of protest from the administration. It had the strong support of many of the most conservative members of both parties, who failed to find anything "extreme" about continuing a law that has enfranchised literally millions of people.

But Reagan's comments suggest that some anonymous authority has again put an idea in his head that could become the basis of national policy.

Presidential misconceptions are dangerous playthings. The notions that inflation is being whipped, that the Soviets are on the verge of starvation, that the dissent in NATO over nuclear policy is just communist propaganda, that the Voting Rights Act is "pretty extreme"--these ideas, volunteered by the president in one brief conversation, are enough to make you wonder.

Where does the president get these ideas? Who tells him these things are so? And who--if anyone--around him is brave enough to disabuse him of these notions?