It might seem a little extreme to compare the Philadelphia of the 18th century A.D. with the Athens of the fifth century B.C., but seldom can any one city have been the center of so much learning, inquiry and innovation. And not just a center -- for work in astronomy, medicine, law and other fields -- but also a magnet. When Joseph Priestley, the virtual discoverer of oxygen, had his laboratory smashed by the mob that shouted "Church and King," he quit Birmingham, England, and removed his scientific instruments and his heretical religious opinions to Philadelphia. So did Thomas Paine, the self-taught customs officer and designer of the first iron bridge. Paine, the moral author of the Declaration of Independence, was lucky in his timing but also lucky in his patronage. Across the Atlantic, he bore with him a letter of recommendation from Dr. Benjamin Franklin in London. And if Philadelphia was Athens at all, then Franklin was, as well as its senior citizen, its Socrates.

In how many dimensions can one observe this figure, on his tercentenary? Unlike most philosophers, he was also an eminently practical man, schooled at first in the most charming and useful of trades -- that of a printer -- but wise in the ways of business and some distance ahead of his time in matters of science. If he did not exactly discover electricity, he did establish beyond doubt that it was a principle at work in the natural universe. And for him, discovery of this kind was intuitively linked to the possibility of the useful: for the lightening of the human load and, more important, the enlightening of the human mind.

Unlike most revolutionaries, he was a conservative. He did not, for example, join Benjamin Rush and Thomas Paine in the Anti-Slavery Society until quite late in his life. I think it may have been John Maynard Keynes who observed that conservatives often make very effective revolutionaries, in that they have tried to make the existing system work and have come to understand very clearly why it must be changed. Benjamin Franklin offered to pay the damages of the Boston Tea Party. If the British authorities had not treated him in such an arrogant and underhanded manner, and had not had such a paltry idea of the man with whom they had to deal, he would very probably have negotiated a brilliant settlement of the outstanding disputes between the colonies and the motherland. This was certainly his wish. But as it was, his full talent as a diplomat was only disclosed when he became the first and best envoy of the American Revolution. (He never lived to see the full effect of the French one.)

One ought, also, to remember his physical courage and his readiness to take risks. He very nearly died on a hazardous expedition to French Canada during the fighting in 1776, and repeatedly stood the danger of first-hand experiments with lightning, which on at least one occasion could have cut his life extremely short. His insouciance about all this must bear some relationship to his dry and highly developed sense of humor. The Founding Fathers were not to be renowned for their joke-cracking capacities: One may page through Thomas Jefferson's elegant correspondence and yet become dispirited by the want of a jest. You can never be sure exactly when Franklin is joking: In the "Autobiography" he boasts with Abramoff-like glee that he both recommended an increase in paper money to the Pennsylvania Legislature and then eagerly received the contract to print it. But in any crisis of seriousness, Franklin was also the main man. He was drafted onto the committee that drew up the Declaration (and may well have been the one who imposed the ringing term "self-evident," as against the more pompous "sacred and undeniable" in its crucial opening stave.) When George Washington's horse bore him into Philadelphia for the grueling meeting that would eventually evolve the United States Constitution, it was at Franklin's front door that the president necessarily made his first stop....