Chapter two of Cutting for Stone, by Abraham Verghese, is titled "The Missing Finger," and we learn how Dr. Thomas Stone, a surgeon, lost one of his fingers.
As a surgeon, Stone was famous for his speed, his courage, his daring, his boldness, his inventiveness, the economy of his movement, and his calmness under duress.
One day while performing a surgery, his assistant is nervous and about to faint, so Stone tries to take her mind off the surgery by telling her the story of his missing finger. He says he
returned to the operating theater the very next day after performing a ray amputation on [his] own finger.
He got an infection in his finger during a surgery and, despite his medical precautions, the infection started to spread; so he decided quickly to get rid of his finger before he lost his life.
Now, five years later, anyone who looked at Stone's hand would barely notice that his index finger was missing. It just looks like his other fingers have slid over a bit to fill the gap. It was a bold, masterful piece of surgery. As it turns out, this new configuration of his hand has worked to his benefit as a surgeon.
The answer to your question is that Thomas Stone deliberately amputated (cut off) his own finger.