In the natural world, animals respond to danger in various ways. Some run and hide. Some stand and fight. But some of the most awe-inspiring do neither. Lions , grizzlies, porcupines - they just don't seem to get threatened.
In the intellectual world, human beings have developed differing responses to the threat of discomforting ideas. Some run and hide - imagining, in keeping with those once-popular see-no-evil-hear-no-evil monkey statuettes, that the unknown can't harm them. Others stand and fight - lashing out violently against all concepts foreign to their own.
But the most formidable do neither. Sure of their own strengths, they simply don't become threatened by ideas.
American democracy, with its emphasis on free speech, seems well fitted to produce citizens of the latter stripe - thinkers capable of assessing a wide range of ideas without feeling threatened by them. So it is particularly disturbing to see evidence from several quarters that ideophobia - the fear of ideas - is on the rise.
The signs come from both ends of the political spectrum. Last month in Washington, the Reagan administration came under fire at a conference on ''free trade in ideas.'' Organized by the American Civil Liberties Union, it highlighted the increasing use of the McCarran-Walter Act (passed during the McCarthy era to keep communists out of the United States) to prevent certain foreign writers, editors, and speakers from entering the country. Under its provisions, Mexican historian Carlos Fuentes, South African poet Dennis Brutus, Colombian novelist Gabriel Garcia Marquez, and Polish poet Czeslaw Milosz have at one time or another been denied entry because of their political beliefs. In 1983 the State Department refused visas to Nino Pasti, a former NATO general and Italian senator opposed to stationing Pershing and cruise missiles in Europe, and to Hortensia Allende, widow of the Chilean President. Both had been invited to speak to US groups. Last year, too, the Justice Department stamped a ''political propaganda'' label on three Canadian films dealing with nuclear war and acid rain before allowing them into the country.
The Reagan administration, however, has also been the victim of this resurgence of ideophobia. Defense Secretary Caspar Weinberger's talk at Harvard last fall was so severely disrupted that he could barely complete it. Ditto for UN Ambassador Jeane J. Kirkpatrick's talk earlier that year at Berkeley: She canceled a second speech and was later prevented by students at Smith College from appearing as their commencement speaker. Three years ago a talk by Central Intelligence Agency Director William Casey at Brown University was interrupted by students in the audience reading aloud from Lewis Carroll's poem ''Jabberwocky.''
These and similar incidents form the basis for a thoughtful essay by Harvard president Derek Bok on the question of free speech on campus, published last month in the Harvard University Gazette. ''When,'' writes Mr. Bok, ''is heckling an interference with free speech and when is it simply a means by which an audience communicates its disapproval?''
It is not a simple question, and Bok treads a judiciously middle course. The university administration is responsible, he rightly observes, for ''maintaining an environment in which free expression and debate can flourish.'' But he warns that ''free speech will not survive in an environment in which many people are indifferent to its existence or hostile to the expression of unpopular ideas.'' At heart, he says, lies the ''critical question'' all proponents of censorship must face: ''Whom will we trust to censor communications?'' And he quotes former University of California president Clark Kerr: ''The university is not engaged in making ideas safe for students. It is engaged in making students safe for ideas.''
True for universities - and true also for nations. The safety of the citizenry, as neo-censors on the left and right need to remember, is born not of fear of ideas but of courage in facing them. Does that mean there are no bad ideas? Not at all: Some are simply awful. But the leadership of a nation that values intellectual courage does not need to protect its would-be thinkers from bad ideas. It needs to encourage them to wrestle with them - to learn to discriminate between the good and the bad. It does not teach them to flee, or to lash out violently: It seeks instead to develop in its citizenry the serene confidence of mature judgment.
In the end, those who shout down Mr. Weinberger and those who shut out Sr. Fuentes are cut from the same cloth. They need to remember that Descartes's famous Je pense, donc je suis means ''I think, therefore I am'' - not ''I think, therefore I fear.''