When he opened fire on President Reagan, the accused assailant, John W. Hinckley Jr., raised again the question: How can the United States protect itself against one of its most cherished dreams?

The question has been put before, as recently as last Christmas in New York, when Mark Chapman killed John Lennon. Almost certainly the question will be asked again by somebody else with the price of a secondhand gun and the yearning to enter, however briefly, the sacred grove of celebrity.

The dream might once have been Jean Jacques Rousseau's: a romantic panorama of man as noble savage at play in paradise, of man set free from laws and schools and institutions, free to constitute himself as his own government, free to declare himself a god.

Or to define himself by any other name that might come into his head. Edward M. Richardson, the would-be assassin arrested in New York on April 7 while on his way to Washington to complete what he called "Hinckley's reality," styled himself "Interrogator, People's Court."

Although easily corrupted and almost always misconstrued, Rosseau's dream came to be an article of the American credo. Yet from the beginning, Americans set the ideal of a primitive association of gods and heroes in opposition to the idea of a civil government conducted by mere mortals. The older, Roman idea of a republic recommended itself not only to Jefferson in his more federalist moments but also to Washington, Jay, Hamilton, Adams and everybody else in Philadelphia in the summer of 1787 who held "natural man" in fairly low esteem. The authors of the Constitution put their faith in the vices rather than the virtues of their fellow citizens.

Since the fire at Hiroshima, the argument has shifted in favor of primitivism, magic and Rousseau. The pagan gods have been restored. More terrible and omnipotent than in the dear, sweet days before the death of Christ, the old idols have now discovered that their powers of creation and destruction have been much augumented.

Consequently, politicians with access to apocalyptic weapons receive the adulation owed to despots, or the propitiation due to priestly servants of savage gods. Popular adoration of divine images has become so habitual that people find it easy to accept celebrities (political and otherwise) enthroned in a broadcasting studio on Mount Olympus, conversing with one another in an eternal talk show.

Most celebrities are as acutely conscious of the subjugating power of fame as Rosseau was. His writings allude constantly to his desire to walk into a room and seize the instant and universal approbation of everyone present, to focus on himself all eyes, all praise, all attention, all sexual feeling. No doubt Henry Kissinger would understand what he meant. So would Abbie Hoffman. So would the literary critics who take pleasure in assassinating one another's books.

It is conceivable that nobody attempted to assassinate President Nixon or President Carter because neither of these sober gentlemen inspired devotion; both demonstrated an emotional inadequacy so palpable as to quiet all the provinces of the id. Reagan, on the contrary, seemed to be having such a god time: a smiling man, laughing at prerecorded Hollywood jokes, feeding on jelly beans, and all the while cheerfully withdrawing food stamps from the poor and chatting amiably to the press about his arsenal of hideous weapons. Like other public men attacked by assassins in the last 20 years, Reagan presented himself as a bringer of bad news who wanted to be loved for his trouble.

Who knows what Hinckley had in mind while he waited for Reagan to come out of the Washington Hilton? But it is safe to assume that he had watched a lot of television and had accepted the symbolism of political theater as a literal rendering of the world. His only possessions were a television set, a guitar and a gun. Every important event he'd ever seen, he'd seen on television. Wandering through a landscape of hotel rooms, a noble savage unnoticed by the management, he may have come to think of himself, in Justice ' Holmes' phrase, as "a puny anonym."

Maybe he would have been content with an appearance on the Johnny Carson show. Whatever his reasons, they would have made sense to Rousseau. If, as Andy Warhol foretold, the media will make everybody famous for 15 minutes, what is to prevent a boy from growing up with the ambition not of becoming the president? The latter ambition is certainly easier to achieve: less expensive and more consistent with the educational requirements set forth in the federal guidelines.