THIS IS NOT an age of reason. The Jonestown massacre was an event perfectly typical of the epoch: You can fool most of the people with absolutely any nonsense, all the time, and a belief that the mercenaries were coming for them out of the jungle, that the only escape was mass suicide, was completely normal.

The climax of Leni Riefenstahl's film of the Nuremberg rally comes with Hess shouting to the frenzied faithful, "Hitler is the Party, the Party is Hitler," just before the Leader appeared. No doubt about it, if he had ordered the SS to pass around the Kool-Aid, all those crewcut Nazis would have tossed it back with the same fervor with which they cheered Hess' ravings.

To a large extent, in fact, the latter part of the Second World War was a case of mass suicide: Everybody in Germany knew that the war was lost, and that going on fighting was merely a good way of getting killed. They followed their Leader, all the same, their eyes open, right to the end.

These are modern instances of irrational beliefs which led to catastrophic consequences. There are plenty of other, less dramatic examples of directly harmful consequences of a suspension of the law of reason (the Manson family, for instance, or the SLA), but most contemporary aberrations are harmless Nobody, so far has been eaten by a Big Foot.

But reason is what brought man out of savagery, and its loss can send us back there. Politicians, teachers and writers have the duty to discriminate between sense and nonsense, to point out the difference and to stick by it. The fact that far too many of them, in every country, do nothing of the sort encourages the spread of all kinds of irrationality, including the most dangerous.

The French claim to be a rational people (Descartes and all that), but millions of them, more than in most countries this side of Badylon, believe deveoutly in astrology. That superstition got its start from pre-Copernican astronomers' inability to predict the movements of the planets.

Anything so mysterious and inexplicable, it was thought, must have some anthropocentric significance. Then, 400 years ago, the foundations of astrology were destroyed when the planets' orbits around the sun were first determined. Nevertheless, all these apparently intelligent Cartesians continue to believe in total rubbish, resolutely ignoring four centuries of science.

The Mormons, of all American religious sects, are probably the most admirable and successful. They have all the civic virtues, and they do not hide from their times. However, the Mormon history, as described by the Book of Mormon, strains the credulity of nonbelievers.

Unlike other mythologies, which sensibly avoid precise dates and places for their narratives, the Book of Mormon claims to describe real cities and nations in American which were overthrown. The faithful could have argued a century ago that those colossal ruins might still be found, somewhere, in the remoter parts of the continetn. To do so now would be ridiculous, but the church and its doctrines continue to flourish and to build temples on the Beltway.

The world is full of strange faiths and, fortunately, most people can usually discriminate between the religious beliefs such as Mormonism (or orthodox Christianity, for that matter) and the real fantasies such astrology, homeopathy, the Loch Ness Monster or the Bermuda Triangle. People capable of believing in this sort of nonsense can be supposed capable of believing anything -- and if they did not happen to believe in Bishop Jim Jones, that was because they had the good fortune never to meet him.

But what about all those sensible peopel, nearly half the adult population, who smoke? On any strictly rational examination of the evidence, anyone who smokes is crazy.

But any so sweeping a generalization must collapse under its own weight. Smoking is certainly foolish but, paradoxically, although clearly more harmful than many completely dotty beliefs, it is not so much a surrender to folly, like a belief in astrology, as another illustration of man's refusal to regulate his conduct by cold reason.

THE PRESIDENT of the United States was asked during the election whether he believed in the literal truth of the Old Testament. Like a good Baptist, he said that he did.

We must take him at his word (he promised that he would never lie) and assume that apart from being a nuclear engineer he believes that the sun stood still in the heavens when Moses raised his armsm that the waters covered Mt. Ararat (16,945 feet), that Methuselah lived 900 years and that Jonah was swallowed by a whale.

This is religious credulousness and has been part of humanity's behavior since time began. But what are we to make of that distinguished English astronomer, Fred Hoyle, who claims that Stonehenge is a prehistoric observatory? We put him in the same category as Newton, obviously, who spent his declining years working out the mathematical implications of the Book of Revelation.

Other forms of credulousness are born of prejudice. Back to France: A couple of generations of left-wing indoctrination in universities has instilled a profound anti-Americanism into the French, who are therefore perfectly willing to believe anything at all about this country. They have the most perfect belief in the various Kennedy assassination theories, and took Vietnamese communist propagnda for gospel, not on its merits but becauseit was anit-American.

The English are not much better. A Regius professor of history (not Trevor-Roper) at one of our ancient universities was asked once why he believed that Robert Kennedy, as attorney general, should have connived at the covering up of the Lyndon Johnson/CIA murder of his brother, which the professor believed in. He replied, "Oh, he's got presidential ambitions, you know, and so he has to play the game."

Political passion frequently leads to a startling suspension of sense. An Irish priets, now a bishop in Texas, once returned from a visit to a prison in Belfast to assure a press conference here that it was worse than Dachau, which he had seen as a chaplain with the U.S. Army in 1945.

The assembled reporters were unable to sway the holy bishop from his comparison, despite its evident absurdity (murders in Dachau -- about 50,000; murders in Long Kesh -- about none). Sancats simplicitas .

I doubt that such purity of motive should be extended to cult leaders, like Jim Jones, Sun Myung Moon, those ridiculous Indian gurus or Lyndon LaRouche. This last directs the National Caucus of Labor Committees and his followers, as fanatical as any Moonie, believe in his vision of a world conspiracy directed by the Rockefellers which controls everything from the SLA to the PLO by way of the White House.

But at least all these people believe in themselves or thier doctrines before they set out to convert the multitudes. What about those who set out to dun the public, propounding some new nonsense in the certain knowledge that somewhere out there will be enough people to buy the product book or TV show and make a profit for the happy inventor?

About 15 years ago a French charlatan claimed to have found a cure for childhood leukemia (then an always fatal diseas). The "cure" involved a special microscope and secret potions. He opened a clinic in Corsica and was making a nice, discreet living out of it until, for some reason, the popular press in Scotland got hold of the story.

They splashed it all over the front pages, without ever saying that they actually believed in it, but people do not read newspapers carefully. Before long money was being raised, rallies were being held, Scottish children were being packed off to Corsica, the government was denounced and a great agitation arose to give the man a clinic in Glasgow.

The bubble burst in the end, killed by one of the serious papers in fact, the children died and the papers turned to other things. They never admitted that they had knowingly decived their readers and the unhappy parents for entirely corrupt purposes.

THERE IS NOW a national agitation here to allow the sale of a miracle cure for cancer, laetrile, and the armies of sufferers are being misled by the gullible and also exploiterd by quacks, charlatans, demagogues and newspapers who know perfectly well that it is a fraud and a cruel deception being practiced upon the dying.

It is all very wrong of them, but in a society as credulous and superstitious as ours, who can really blame them? The problem is why this scientific age should be as credulous as it clearly is.

The sheer incomprehensibility of modern science is part cause. Lord Snow long ago pointed out the dangers of the two cultures, in which those who understand the second law of thermodynamics have little in common with the rest of us, who don't. Only those who don't can believe in UFO's, which are scientific nonsense, as opposed, for instance, to Atlantis, which is generalized nonsense.

The History Book Club has recently given its imprimatur to a book claiming that Atlantis was peopled with Celts, and sank beneath the waves off the coast of Denmark. It is possible that if the club's directors knew about thermodynamics they would have been more skeptical, but I doubt it.

Then there is a widespread distrust of science and scientists. Many of the things scientists have done and are doing -- building atomic bombs or recombining genes -- are so terrifying that many people take refuge in health food and batik. A scientific Enthusiast can be just as dangerous as your run-of-the-mile cult leader.

The traditional view is that it is all Rousseau's fault. He turned his back on the Age of Reason (epitomized by Voltaire) and led Europe down the Romantic road to ruin. It might be more sensible to assert that science has had three or four centuries to convert mankind to reason and that its evident failure is proof that it can't be done.

This is perhaps the natural condition of the species. Rational moments like the 18th century are exceedingly infrequent, and man's innate credulousness is evidently far stronger than any amount of education or scientific progress.

The first rule still apphes: When in doubt, blame the schools. The second rule, equally sound, is blame the press (television is beyond redemption). Even though scientists, teachers and writers cannot really hope to extend the realm of light very far, they must keep trying. It is the only way to keep out the darkness, which is full of mercenaries, including Jim Jones and Hitler.