For centuries, people have traveled to Rome to visit the Sistine Chapel to stare up at Michelangelo's stunning ceiling.
To most, the frescoes represent familiar stories from the Old Testament -- "the Creation of Adam," "the Temptation of Eve" -- and define the way many imagine these characters, even if they've never set foot in the chapel. But there are those who look at the 500-year-old frescoes and see something different.
"[The ceiling] has so many layers of meaning upon meaning, and most of it, if not all of it, is from the Jewish tradition," said Roy Doliner, a Vatican tour guide.
He and Rabbi Benjamin Blech, associate professor of the Talmud, Yeshiva University, have written a book, "The Sistine Secrets: Michelangelo's Forbidden Messages in the Heart of the Vatican."
They say that Michelangelo embedded powerful and even dangerous messages in the frescoes of the Sistine Chapel, that he encoded these messages using his knowledge of mystic Jewish texts and that he intended some images as insults to the pope.
The authors think that Michelangelo would have been exposed to Jewish teachings in Florence, while living in Lorenzo de Medici's household.
Medici was the powerful leader of Florence, and used his influence to bring many great thinkers and artists to his city. Some of them studied the Zohar, a book at the center of a form of Jewish mysticism called Kabbalah.
According to Doliner and Blech, Kabbalah is the key to cracking the code of many of Michelangelo's hidden messages. But their first clue didn't come from an art historian. Rather, it came from a tourist from Indiana who looked up at the famous panel of "The Creation of Adam," and was reminded of something else.
"In the late 1970s, a surgeon went into the Sistine Chapel, took a look at this [fresco]. He said to himself, wait, this is Anatomy 101.… This is actually a cross-section of the brain, the right hand side of the brain," Blech said.
"Now, what's interesting is in Kabbalah we have different kinds of wisdom," said Doliner. "The right side of the human brain in Kabbalah means wisdom -- chokhma." God is appearing from the right side of the brain in that fresco.
Experts agree that Michelangelo would have learned about many different philosophies that captivated 15th century Florence, including Kabbalah, but they don't believe that Kabbalah motivated the iconography on the ceiling.
"One of the great desires of the Renaissance was to reconcile the great religions, the great knowledges of the world, and bring them together and realize that there's sort of underlying wisdom behind all of them," said Michelangelo scholar William Wallace, a professor at Washington University in St. Louis.
Wallace suggests that there are reasons for the brain image in Christian theology.
"After all, God is all-knowing," he said. "So in a sense a brain is an appropriate way to suggest the greatness of God … although one of my students in class said it that looked like a stomach."
Doliner and Blech are not the first to see Kabbalistic imagery on the ceiling. In 1986, Jane Schuyler, an art historian and professor at York College, CUNY, published an article about the beautiful woman under God's arm in the "Creation of Adam" panel. Many assume this character is a preview of Eve, but Schuyler thinks this mystery woman could represent Shekhinah, God's female beloved from Kabbalah.