Along with the help of a former Yale researcher, a longtime admirer of Leonardo da Vinci is trying to challenge commonly accepted beliefs about the life of one of the greatest minds in history.

After mistakenly hearing a museum tour guide refer to da Vinci’s left hand as “sinister” — when she actually said “il sinistra,” meaning left-handed — Ben Sweeney became interested in the idea that da Vinci may have had a misshapen left hand, a theory that he said has been overlooked for 500 years by the art world. Sweeney, a real estate broker in San Diego, started researching da Vinci’s sketches, paintings and mathematical formulas, questioning accepted knowledge about da Vinci that he feels is not held up by factual analysis.

“I’m coming into the Leonardo world as an outsider,” Sweeney said. “Many people in academia run the risk of getting locked into their ideas and stuck in their own perspective. I don’t come in with anything preconceived.”

Although Sweeney’s research runs the gamut on da Vinci — including art, science and math — his study on da Vinci’s anomalous left hand has been the subject of most interest. Through analysis of different da Vinci sketches, Sweeney developed the idea that the third and fourth fingers of da Vinci’s left hand were connected. This condition is called syndactyly, in which two or more digits are fused together.

Sweeney said that art historians have noticed issues with da Vinci’s hand in his sketches before, but it was always written off as the result of a stroke da Vinci had late in life. But Sweeney took his theory to Joseph Upton ’66, a former Yale-New Haven Hospital surgeon who researched at the University, who agreed that it appears to be a syndactyly hand.

Upton, a current Harvard Medical School professor, said that although the theory may not be able to be verified — proof would involve an X-ray — he does believe Sweeney’s conclusion that the anomalies in Da Vinci’s work are not simply a result of a stroke.

The theory also looks at the depiction of Jesus’ left hand in “The Last Supper” and “Madonna with the Yarn-Winder.” Jesus’ left hand is misshapen in both of these paintings, Sweeney said. In “Madonna with the Yarn-Winder,” the disfiguration was discovered through the use of infrared technology, Sweeney said, and adds to his theory that da Vinci had some sort of obsession or preoccupation with syndactyly hands.

“Whenever the art world came across a misfigured hand, they considered it a mistake,” Sweeney said. “In my opinion, da Vinci didn’t make mistakes. He’s definitely portraying syndactyly hands.”

Robert Goldwyn, a professor of surgery at Beth Israel Deaconess Medical Center in Boston, said Sweeney is probably on the right track about da Vinci’s hand. The theory is interesting because Sweeney is looking at da Vinci in a way that nobody has in the last 500 years, Goldwyn said, most likely because of Sweeney’s unique position outside the field of academia.

But while some scientific experts agree with Sweeney’s theory, the art world is less open to the idea. Some art historians have said that until conclusive evidence can be found, they see no reason to believe that da Vinci had an anomalous left hand.

“One art historian told me he never wanted to see me again after hearing my theory,” Sweeney said. “The art world wants to assume beauty over truth, and Leonardo knew that. He understood our mental state, even 500 years ago.”

With the help of experts such as Upton — who he is collaborating with on four different anatomical studies — Sweeney is working on about 40 different studies ranging from an examination of da Vinci’s health to the development of a mathematical form with which to study da Vinci art from a mathematical perspective. One of his most recent projects was the creation of a robot that da Vinci had created full plans for in his notes, Sweeney said. The “lost robot” has been traveling the country on tour and is currently on exhibit at the University of Tulsa in Oklahoma.

Sweeney said that he hopes the research will culminate with a book written at about the fifth grade reading level. Getting the information out is a slow process because data can be misconstrued if its not presented clearly enough, he said, but he expects the results to make an impression on the art community.

“This is going to be the control-alt-delete of the art world,” Sweeney said. “There will be a concordance of his work, but this is really just the beginning.”