ONE MAIN AUTHOR of our Constitution died more than 30 years before it was written. When the first Congress convened in 1789, it put into effect a system of laws largely taken from the writings of the French nobleman Montesquieu (1689-1755), a man of privilege who insisted that power properly belongs to the people. The tricentennial of the birth of this neglected "godfather" of the Constitution is being celebrated in an exhibit at Gunston Hall, which appropriately enough was the home of George Mason, a similarly neglected contributor to the central document of the American republic. Key constitutional concepts, especially the separation of the judicial, legislative and executive powers, came from Montesquieu's "The Spirit of Laws," published in 1848. It was the magnum opus of the doubly titled Charles-Louis de Secondat, Baron de la Brede, Baron de Montesquieu, and many of the drafters of the Constitution knew long passages of it by heart. Montesquieu had no patent on the idea that freedom is the natural right of all, but his writings gave inspiration and practical shape to the sentiment for liberty that was abroad in Europe and America. There were no perfect men and so there could never be a perfect government, he said. The thing to do was to strike a balance between the right of people to be left alone and the necessity of government to maintain order. Mason, author of the Virginia Declaration upon which the Bill of Rights is modeled, was a close student of Montesquieu, as was Thomas Jefferson, principal author of the Declaration of Independence. James Madison, who dominated the Constitutional Convention of 1787, could quote Montesquieu till the cows came home, and both the Federalists and anti-Federalists cited chapter and verse from the Frenchman's works as debate raged and details were decided. Montesquieu often contradicted himself, and alternated between gloomy forebodings of the inevitability of despotism and brighter, though guarded, delight in the innate nobility of man. The happier view predominated, and anyway it was Montesquieu's spirit even more than his words that influenced the men who were inventing the world's first constitutional democracy. It is altogether fitting and proper that the tricentennial of Montesquieu's birth falls on the bicentennials both of the first American Congress and of the French Revolution. His countrymen, alas, do not appear to have read Montesquieu as carefully as did our founding fathers, and failed to build into their Declaration of Rights the checks and balances that might have kept the heady dawn of French freedom from degenerating into the Terror. The Gunston Hall exhibit was designed and personally installed by Baronne Charles-Henri de Montesquieu, wife of the eighth-generation grandson of the great philosophe and mother of the next baron-to-be. Miho, as she calls herself, is president of the nonprofit Montesquieu Foundation, which is headquartered in Washington and seeks to keep his spirit alive. The Montesquieus still live in the Chateau de la Brede, and many of the portraits, books, manuscripts and memorabilia in the show were drawn from the estate library and archives. The exhibit is necessarily a complex one, for Montesquieu was a man of many parts. But his spirit is easily absorbed, and his cautious optimism is compelling: An ancient philosopher compared the laws to spider webs which, strong enough to catch fleas, are torn apart by birds. I would compare good laws to those large fishing nets in which fish are caught even though they believe themselves to be free, and bad laws to those nets so tightly meshed that everybody immediately senses he is trapped. MONTESQUIEU: THE MAN, THE MIND AND THE SPIRIT -- Through March 30 at Gunston Hall, on Mason Neck in Fairfax County (take exit 55 from I-95 and follow the signs). Open 9:30 to 5 daily. Admission $3 adults, $2.50 seniors, $1 children 6 through 15. Good wheelchair access to exhibit center, but the historic plantation house has high steps and no ramp. 550-9220.