We know our brains are important to our overall physical and mental health. But a man in France is challenging commonly-held medical beliefs about how much of our brains we need to survive. The man — whose identity has been withheld to protect his privacy — is missing roughly 90 percent of his brain, yet he leads a perfectly normal life complete with a wife, children and a government job.

According to a study first published in the U.K. medical journal The Lancet, the 44-year-old Frenchman went to the doctor complaining about weakness in his left leg. When doctors could not find an orthopedic cause for his symptoms, they used magnetic resonance imaging to take a closer look at his brain. They were shocked to discover that it was almost entirely missing.

After a more thorough medical history, the doctors — including Lionel Feuillet, Henry Dufour and Jean Pelletier — learned that their patient had been born with hydrocephalus, a condition that causes a buildup of fluid in the brain. The man had been treated for the condition as an infant with the insertion of a shunt in his brain to drain the excess of fluid. The shunt stayed in place until doctors inexplicably removed it when he was 14.

Doctors believe the man continued to produce excessive fluid in his brain, fluid that over the next several decades eroded his brain until all that was left was a thin perimeter of brain tissue.

That explains what happened to his brain, but it doesn't explain how he could function so well without it. How was he able to talk, move and solve problems without a fully functioning frontal lobe, the area of the brain that typically controls these actions? How was he able to remember anything without a complete hippocampus, to control his body temperature without a full-sized hypothalamus, or to walk without a normal cerebellum?

Testing revealed that the man had an IQ of 75, which is lower than the average score of 100, but not low enough to be considered mentally challenged or disabled. Rather, this man appeared to be living a relatively normal life. He was married, had two children and was employed in what the doctors described as a "white-collar" government job. All of this with only about 10 percent of a functioning brain.

One theory that explains this man's remarkable life is that of brain plasticity, the idea that the brain is able to reorganize itself to allow functioning areas to take over the jobs of the sections of the brain that are not functioning.

It's a rather incredible idea that challenges our whole notion of the brain and how it works and which parts of it are necessary for survival.

In this man's case, the doctors believe it was the slow erosion over time that allowed the brain to continually build new pathways and to keep functioning at a normal level.