If Henry Lepage were to design his own signet ring it would probably consist of a boomerang rampant, since he thinks government do-good programs invariably backfire.

His new book, "Tomorrow, Capitalism," has sold very well in Europe and is now issued here, and since it was written for laymen you will not want to say you cannot make heads or tails of it.

"What do they say, kindly fascism?" he said.

"Friendly fascism," I corrected the buzzword for him.

"Well, that's what some say about my book," he said. Lepage has rather light brown eyes and silky brown hair that looks as if it might go red in the summer, and if he were a dog you would immediately suspect a cross of a Welsh terrier and a St. Hubert's hound. Very straight, very solid, all those good things.

"There are two fallacies we keep making," he said, and I wondered if he was going to say no, not a Welsh terrier at all, but that was only because I had been browsing in a splendid picture book of dogs. He had not. This may be the place to observe the obvious (which Lepage often insists on), that one views the world through his own interests, not necessarily selfish interests; indeed they may be quite noble interests, but we do not get away from our own brains and senses, of course.

The fallacies, to get back to them, are these:

We call on government to correct a problem. "We think government is motivated by what we call public interest," he went on, "sometimes forgetting that their conception of public interest is intimately related to their electors."

In other words, government is interested in votes. Even more, sometimes, than in "public interest."

The second thing we do, Lepage is sure, is assume that political fiddling with our lives is less selfish than big business or free-market fiddling. Not so.

Since Lepage quickly saw he had his work cut out to discuss economics with an economical illiterate, as it were, he summed up the nutshell:

"Every time government wants to do something good, it does it badly. Whenever the free market undertakes something that may even seem bad, it winds up good."

That was sufficiently black and white even for me to grasp, so I inquired about free public schools, a typical example of government aiming to do lovely things but which (as Lepage insists) winds up with an opposite result.

"In France, a university education is free, and that's a very positive social thing, right? But look at it, if you are poor your parents want you to go to work as soon as possible. So the guy who is not in school is paying taxes, some of which subsidize your university education.

"When you make a cost analysis over a lifetime, you find that the least well off are paying a greater percentage of these costs: If you go to the university you start work at 25 and retire at 65. The unschooled guy dies three or four years before the educated guy (Lepage says that's how it works out in France) so his retirement benefits do not go on as long, and since he earns less his retirement benefits are less to begin with, and you see that the worker pays relatively more than the middle-income guy.

"So here is something that seems on the surface to be highly social--to benefit the low-income group--but which in fact is regressive.

"Or take health. In England health care is free, and it's supposed to benefit the poor. But in fact the mortality rate of the poor has increased. Since health service is 'free,' there is a greatly increased demand. Doctors are paid more if they see more patients. Time spent with an individual is less than it was. The rich go to doctors outside the free-health program and do not suffer. So the system results in the opposite of what was intended."

Lepage thinks we'd do much better if we paid for what we got. All kinds of things get done that people don't really want, as a society, but which a bureaucrat or lobbyist may want."

It's like those asinine and dangerous bumps on 46th Street, I gathered, that nobody in his right mind wants, but which the District government spends thousands of dollars to install on the general theory that the traffic is greatly improved if the streets are made into obstacle courses. That sort of brains, I gathered, was what Lepage was talking about; whereas if everybody had to chip in a few bucks knowingly to install the bumps, nobody would do so.

But suppose (getting back to free public schools) you agreed with Lepage that the present public schools are inefficient, vastly too costly, and unproductive of that freedom of opportunity they are supposed to guarantee, even so, how would you change the system?

"You start handing out vouchers to be used for any sort of education the parents want, or you start handing out tax rebates for private schooling, and you're going to be in trouble with the courts and with all those people who would see this as a ruse to buy good education for your own kids while leaving the poorest with the utter scrapings of the barrel," I said, "and first thing you know we're in a big national hassle about the Roman Catholic Church and so on and on. There's enough prejudice already without feeding it."

Lepage said, however, that people seem to find ways around government schemes they do not have faith in. Thus, come to think of it, the public school enrollment of Washington is almost all black, somewhat the reverse of the integrated schools originally intended.

He set off to speak to the Heritage Foundation, a tax-exempt public policy research institute or a right-wing think tank, as some call it. His tour promoting his book is being paid for by the Institute for Humane Studies, Menlo Park, Calif. That institute used to publish books of a generally conservative sort, but wound up with a lot of them on their hands, not having awfully good distribution techniques, so now they look about to find a book they approve of and use some of their money to promote it, instead of publishing it themselves.

I sensed Lepage is rather an admirer of President Reagan.

"You can probably answer this," I ventured. "Why did so many people vote for him who were certain to get it in the neck--unemployment and so on--from his policies? Why did so many people vote against their own interests?"

Lepage, looking rather more like a springer than a nice Welsh terrier, I thought, said:

"Maybe they are a better judge of their interests than you are."

Some Gauls have a lot of gall.