“Dad, hold still.”

As we entered the hospital room that morning, our patient’s daughter was attempting to give him a shave. He was bed-bound after his operation and had grown a salt-and-pepper stubble.

A week earlier, his wife had brought him to the emergency room . He was behaving oddly, mumbling nonsensical sentences and stumbling through the house. Sixty-two years old, male, Caucasian, new and profound neurological symptoms. An M.R.I. of his brain seemed redundant but confirmed the diagnosis: A four-centimeter malignant tumor was invading his right frontal cortex, the seat of his personality, where “Dad” lived.

I’m drawn to the human brain, its unforgiving and protean nature. Just five minutes without oxygen, and the brain loses function. The occipital cortex processes visual information and allows us to see faces, trees, the stars. However, in a young child who becomes blind, as with Helen Keller, this same cortex can be repurposed for entirely distinct functions, like language processing. Early astronomers looked to the heavens for answers. But in the human brain, a three-pound ball of fat, there resides enough mystery and potential to have satisfied Galileo, Kepler and Brahe.

And so I found myself, on what had now been a four-year foray toward a career in neurosurgery, helping care for this patient. I was the sub-intern at a hospital away from home for the month. It was my first week on the job.