1. King Arthur

The protector of Camelot is one of history’s most well known monarchs, but many scholars believe his story to be a legend on par with the Sword in the Stone. The brave King Arthur is traditionally described as having repelled a Saxon attack on Britain in the 5th or 6th century. But while he supposedly won a series of 12 battles against the invaders, the great king is not named in the only surviving history of the conflict. In fact, a full depiction of Arthur did not surface until the 9th century, and an account of Lady Guinevere and the famous Knights of the Round Table only appeared with Geoffrey of Monmouth’s 12th century text “History of the Kings of Britain.”

Even if the modern depiction of Arthur as a knight in shining armor is a myth built up by books like Sir Thomas Malory’s “Le Morte d’Arthur,” some historians still believe these tales were based on a real person. Among other candidates, they argue the Arthur legend may have been inspired by the exploits of the warrior king Ambrosius Aurelianus, the monarch Riothamus or perhaps even a Roman general named Lucius Artorius Castus.

2. Pythagoras

We all learned about the Pythagorean Theorem in math class, but a similarly elegant proof is not available for the existence of its namesake. According to some accounts, the Greek thinker Pythagoras lived during the 5th and 6th century B.C. He is remembered as a philosopher and mathematician, but in ancient times he was better known as the spiritual father of a cult obsessed by numerology, the transmigration of the human soul and—quite bizarrely—the evils of eating beans.

While Pythagoras’ hatred of legumes is well documented, there are no significant contemporary accounts of his life. All references to the great thinker—and perhaps also his famed ideas and formulas—came from his followers, who called themselves Pythagoreans. What stories we do have of Pythagoras are deeply intertwined with myth and the supernatural. One tale describes him as possessing a golden thigh; another declares he was the son of the god Apollo. For some, these lies and contradictions hint that Pythagoras was simply an exaggerated or even fictional leader concocted by the members of a religious sect. Even if Pythagoras did exist, he probably wasn’t the first to discover his famous theorem—evidence shows the Egyptians may have divined the formula much earlier.

3. John Henry

According to a popular American folktale, a burly formerly enslaved man and steel-driver named John Henry once took on a steam drill in a race to construct a railroad tunnel. Pushing his body to the limit, Henry narrowly won the battle between man and machine, only to then collapse and die with his sledgehammer still in hand. This tale of grit and endurance was later immortalized in the folk song “The Ballad of John Henry” in the late 1800s.

The John Henry story is widely believed to have some basis in fact, and a few candidates have even emerged for the identity of its larger than life hero. John William Henry was a steel driver who died during the construction of the C&O Railway in Virginia, but there is no proof that he ever raced a machine. What’s more, records show that he stood only a little more than 5 feet tall—a far cry from the giant described in the legend. Yet another possibility is John Henry Dabney, a former slave who worked on the C&W railroad in Alabama. Witnesses reportedly claimed that Dabney went head-to-head with a steam drill in September 1887, though there is little hard evidence to back up their account.

4. Homer

Scholars have long speculated about the factual basis for the epic poet Homer’s “The Iliad” and “The Odyssey,” but the argument also extends to the bard himself. According to some theories, the greatest of all the Greek writers may not have existed, and even if he did, he is almost certainly not the sole author of his two famous works.

For so influential a figure, there are no contemporary accounts of Homer’s life, which supposedly took place during the 7th or 8th century B.C. He is often described as a blind man who was born on the island of Chios, but even these details are up for debate. This lack of biographical information has led some to theorize that “The Iliad” and “The Odyssey” were actually written by a collection of different poets, or perhaps culled from popular stories passed down orally over generations. If this is true, Homer may have been responsible for first assembling the stories into coherent narratives, but he might also have been a composite figure invented as a way of giving the myths a single author.

5. Robin Hood

Robin Hood looms large in medieval folklore, but are tales of a bandit who stole from the rich and gave to the poor actually based in fact? Sherwood Forest’s most famous outlaw first appeared in poems and ballads in the 14th and 15th centuries, and historical evidence shows that some criminals were known as “Rabunhod” or “Robehod” even earlier. Most of these literary accounts describe Robin as a commoner who led a gang of bandits in defiance of the hated sheriff of Nottingham. However, some subsequent versions reframe him as an aristocrat-turned-outlaw, along with adding many of the story’s most popular supporting characters, like Maid Marian and Friar Tuck.

Researchers have tried to pin down the identity of a real life Robin Hood for centuries, but no clear candidate has emerged. The most popular account describes him as a follower of King Richard the Lionheart, but others label him as everything from the Earl of Huntingdon to a member of the Knights Templar. Still, an increasing number of historians now hold that stories of Robin Hood and his merry men were simply medieval myths that arose as popular fables about resistance to oppression.

6. Lycurgus

Lycurgus is remembered as the man who shaped the Greek city-state of Sparta into one of the most feared military powers of the ancient world. Sometime between the 7th and 9th century B.C., this famed lawgiver is said to have instituted a series of hard-nosed reforms addressing everything from marriage and sex to wealth and childrearing. Perhaps the most famous of these concerned the creation of the agoge, a rigorous, multi-year training program designed to fashion Spartan boys into fearless warriors.

While there is no doubt that the Lycurgan reforms were enacted, historians are still unsure if the man himself actually existed. The Spartans did not record their history in writing, so most of what is known about their most prominent leader comes from later, often wildly contradictory sources. Lycurgus’ biography is also filled with several mythical occurrences—one account claims he ended his life by self-enforced starvation—leading some to speculate that he was merely a god-like figure invented by the Spartans as a way to attribute their culture to the work of a single creator.