If applause is indeed the echo of a platitude, applause probably greeted the legal director of the American Civil Liberties Union when he denounced ''government-imposed ignorance'' op-ed, July 18 . But before applauding, understand that Burt Neuborne's worry is ''ignorance'' imposed by a ban on tobacco advertising.

He thinks such a ban would be unconstitutional and immoral. It would be neither.

In a recent decision upholding the constitutionality of Puerto Rico's ban on the advertising of casino gambling, which is legal there, the Supreme Court reiterated the principle that commercial speech is more subject than other speech to regulation when the regulation directly advances a substantial government interest, such as public health. That principle flows from this one: The First Amendment is, after all, an amendment to a political document. Its primary protection is for speech related to the process by which we govern ourselves -- the working of representative government and the cultural activities that nourish a free society.

In the Puerto Rico case, Justice Rehnquist, writing for the majority, argued that regulation of the advertising of a product is less intrusive than regulating the sale of the product. (There are only a few products, such as contraceptives, the sale of which enjoy constitutional protection.) Smoking, like gambling, is not a constitutional right. It would be perverse to argue that a legislature can ban a product or activity but cannot regulate commercial speech that stimulates demand for the product or activity.

It is odd to suggest that banning tobacco advertisements would constitute ''enforced ignorance.'' Ignorance of what? The availability of cigarettes? Neuborne considers a ban on tobacco advertisements a form of ''censorship'' and says all censorship arises from ''paternalistic zeal to shield others, presumably less enlightened, Americans from speech that one or another pressure group thinks isn't particularly good for them.''

But no one thinks tobacco advertising is ''good for'' someone who reads it. Perhaps because he knows that reasonable people cannot differ about tobacco advertising's potential for good, Neuborne leaps, as the ACLU is wont to do, to the notion that any regulation of any expression puts society on the slippery slope to totalitarianism. ''The hallmark of a totalitarian state,'' he says, ''is a government-managed information flow.''

But examine the average cigarette advertisement -- toothsome young people frolicking in surf, a picture of a mountain meadow, slogans such as ''Get a taste of it'' or ''Come to where the flavor is.'' Try to measure the information content. Cigarette advertisements are not seminars; they are inducements.

Neuborne says it is incipiently totalitarian for government to use ''manipulation'' of information as a ''behavior-modification tool.'' By ''manipulation'' he seems to mean any government use of selected information for government purposes. By his logic, he must oppose government requiring health warnings on cigarette advertisements -- the only real information in most such advertisements.

Much modern illness is voluntary, meaning behaviorally based: People eat and drink foolishly, exercise too little, smoke, drive recklessly. The only substantial improvements of public health immediately available can come not from new medical technologies but from modifications of behavior. The most successful, cost-effective government programs of the last several decades have involved government-directed flows of information -- about smoking, high blood pressure, automobile safety, etc.

And the behavior-modification capacity of advertisements? Without ascribing to advertising more manipulative power than can be measured, one fact is telling. Last year $2 billion was spent advertising tobacco products.

It is, to say no more, counter-intuitive to suggest, as some industry sources do, that the principal purpose of such advertising is to influence the choices of confirmed smokers who change brands. Only 10 percent of smokers change each year. Anyway, many competing brands are manufactured by the same parent companies that are buying the advertising.

Given that 350,000 persons die each year from smoking-related illnesses and 1 million quit, the industry must find 1.35 million new customers a year just to maintain its market. Advertising is designed to manufacture new demand, not just redirect existing demand.

Furthermore, cigarette advertisements, which are low on information and high on appeals to social aspirations, are apt to be especially effective with young people. Sixty percent of today's smokers started by age 13 or 14. The evidence from the nations with the severest limits on cigarette advertising is that after such advertising is limited, adult smokers continue but fewer young people start.

Sen. Bill Bradley (D-N.J) notes that the tobacco industry, having tried the patience of reasonable people by arguing that smoking does not cause illness, now suggests that advertising does not cause smoking. Two billion dollars says it does not believe that.