05/28/2013 11:38 am ET Updated Jul 28, 2013

Figures, Fingers and Rings

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June is soon: the month of new rings on fourth fingers. Did you ever wonder why? I mean, why that finger? Why don't we wed with our thumbs?

Tradition, tradition... we'll never know for sure. But there are some wacky theories out there. I came across three while researching a still-unfinished essay on hand-mind connections in math. They were proffered at a banquet in fifth-century Rome (like the ones we staged in Latin class, with noblepersons slumped on couches, fed by slaves clad in drab?). The writer, Macrobius, who took notes, tells us that the guests that evening included Horus, an Egyptian-born philosopher; Disarius, a physician; and the aristocrat Caecina Albicus. Horus asked Disarius why the fourth finger of the left hand is the one that wears the ring. The doctor explained erroneously that it's because there is a special nerve running from the heart to the fourth finger of the left hand. The ancients called it the vena amoris and honored it with a ring. Caecina Albicus had heard a different story from the pope: In olden days free men sealed agreements with signet rings; the seals were engraved on easily shattered gems. The fourth finger of the left hand was the least likely of the 10 to bump into things. But an Egyptian priest had told Horus that the fourth finger is crowned because, when bent, it stands for the number 6, which is full, perfect and divine.

The dubious Caecina Albicus said to take your pick of the theories. I pick Horus' and stumble into a vanished world -- and into a bramble patch of questions. First bramble: Why is the number six "full, perfect and divine"? "Full" I don't know about, but "perfect" I do; this bit of ancient number theory is still taught today. A perfect number is one that's the sum of its divisors. Which numbers divide 6? The number 1 does; it divides everything. The number two divides six, and so does three (and so does six itself, but we don't count that). Now look: 1 + 2 + 3 = 6. Or, rather, I and II and III make VI; Romans used Roman numerals, and our + and = signs had yet to be invented. As for "divine," so perfect is the number six, said St. Augustine, that God chose to create the world in six days. Surely God was a mathematician! The number 28 is perfect too (I and II and IV and VII and XIV make XXVIII), and lo, the lunar cycle is 28 days long! The third perfect number is CDXCVI, which means something in string theory.

Next, a thornier bramble: What does it mean that, when bent, the ring finger stands for the number six? I'm baffled, but Horus didn't have to explain; all the dinner guests understood. It had been common knowledge since time immemorial and would be for another millennium. Indeed, 700 years after that dinner, Fibonacci insisted that "those who wish to know the art of calculating, its subtleties and ingenuities, must know computing with hand figures, a most wise invention of antiquity, according to its use by the masters of mathematics." That's Fibonacci as in "Fibonacci numbers," but I quote the man, not the rabbits. He said this in Liber Abaci, a book he wrote to teach his countrymen to use "the nine Indian figures and the symbol the Arabs call zephir." Vale, Roman numerals! Farewell! The Hindu-Arabic system is simpler and faster, and you can check your work. But first, said Fibonacci, you have to know "how the fingers must be held in the hand."

"Finger reckoning," as historians call it now, wasn't counting fingers and toes. You signed all the numbers from 1 to 29 with the fingers of one hand! Fibonacci drew a chart to show how. The numbers 1 through 9 were signed with the little, ring and middle fingers. (You bend your little finger to make a 1; you bend the fourth to make a 6!) The numbers 10, 20 and all the decades up to 90 were made with the index finger and thumb. The same signs on your right hand meant hundreds; hands raised or turned meant tens of thousands and so on. And signing numbers was just the beginning. Mathematicians of those days could add, subtract, multiply and divide, check their arithmetic by casting out nines and do fractions on their fingers with the speed of light. This wasn't child's play, nor was it a sport for the dexterously challenged.

Like his medieval Arab teachers, Fibonacci put finger reckoning and the numerals on a par. Finger reckoning "expressed mathematical knowledge complete, whereas the Indian system was reckoned as a technique useful for numeration and calculation." They were complementary, like handwriting and typewriting before the touchscreen. Gradually, over the next two centuries, mathematical knowledge complete was expressed in the Indian system, and then the printing press swept finger reckoning aside. But it lingered in the marketplace into our time. When I was a child, my mother put her foot down: "Don't talk with your hands!" My expansive gestures were a visual madeleine, a glimpse of the pushing, shoving immigrants bargaining in the family shop on New York's Lower East Side. She taught me American manners instead, like shaking hands with grownups and clapping politely at concerts.

As an adult mathematician, I talk with my hands, and I think with them. I gesture when I lecture. I draw diagrams to understand relationships and clarify my thoughts. I learn from building 3-D models, and from the models too. Fibonacci would understand. "Memory and even perception correlate with the hands and figures," he wrote. Does a vena manus link our brains and hands, I wonder? (I'm still working on that essay.)