Henry Marsh spent nearly 30 years as a neurosurgeon, but there are some things about cutting into people's brains that never become routine.
In his new book, "Do No Harm," Marsh vividly describes a brain surgery in which he's looking for a tumor that needs to be excised. "The idea that my [tool] is moving through thought itself, through emotion and reason, that memories, dreams and reflections should consist of jelly, is simply too strange to understand," he writes.
We tend to think of a dividing line between the body and the mind. But a neurosurgeon like Marsh confronts the reality that the mind is something that may well be entirely physical: a jelly-like mass full of thoughts and functions that can be altered or irreparably damaged with a metal tool.
"If I stray into the wrong area, into what neurosurgeons call eloquent brain," he writes, "I will be faced by a damaged and disabled patient."
Marsh acknowledged, in an interview with Fresh Air's Terry Gross, that it's hard to wrap our minds around what modern science has taught us about our thoughts and feelings, to reduce them from something unknowable and unquantifiable to mere electrical impulses.
"My thoughts don't feel like electric chemistry, but that is what they are," he told her.
Still, when it comes to the brain, there are more mysteries than there are answers.
"We can't even begin to explain how consciousness, how sensation, arises out of electric chemistry," Marsh said, "but the fact of the matter is it does."