Calling someone a half-brain or describing a person as thinking with half a brain is often as an insult. But the brain is a resilient organ - there are many cases of people with congenital brain defects or traumatic brain injuries who can still survive even though their whole brain isn't intact.
Some people do actually live with half a brain, as a result of a hemispherectomy - surgical removal of half the brain done to control severe cases of seizures. Some other extreme cases include hydranencephaly, where the entire cerebral portion is missing and all that's present is the brainstem. Moreover, there are nine documented cases of people living without a cerebellum, the part of the brain that controls voluntary movement.
The brain's ability to rewire itself is remarkable. It seems the one piece that isn't dispensable is the brain stem, which controls the heart, lungs, and blood pressure. But although missing certain parts of the brain may be compatible with life, cognition, movement, personalities, and memories can be affected. So it's always advisable to keep our brains whole.
