What are some themes in Arthur Conan Doyle's short story "How It Happened"?

In Arthur Conan Doyle’s short story “How It Happened,” a spiritual medium, able to communicate with the dead, narrates a first-hand report from a wealthy dead man.  The man had owned a brand new luxury car, which was usually driven by his chauffeur. One night, however, the owner decided that he would like to drive the car home from the train station in the dark. Coming down a steep and dangerous hill, the car lost its breaks. Despite the best efforts of the owner and chauffeur to regain control of the vehicle, it crashed. The chauffeur was injured, but the owner was instantly killed, although he didn’t recognize immediately that he was dead. At the very end of the story, he remembers having been greeted by a good old friend of his, whom he knew to be long dead. The owner was shocked to discover that he, too, had died.

Doyle’s story can be interpreted as implying a number of themes, including the following:

• how instantly and unexpectedly anyone – even a wealthy person – can make the transition from life to death.
• how impossible it often is to reverse the consequences of our choices once those choices have been made.
• the idea that wealth and status cannot prevent or assuage death.
• the idea that wealthy and socially prominent people often take risks that negatively affect people poorer than themselves – risks that poorer people, who live harder lives, are often too sensible to take.
• the idea that however horrible death may seem from the perspective of those still living, it may not be as bad as we imagine.  Thus, the dead man’s friend gently asked him, as he lay near the crashed vehicle, “No pain?”  To which the dead man replied,

“None,” I said.
“There never is,” said he.

• the idea that English gentlemen remain English gentlemen even after they die.  :-)
• the idea that our personalities do not change even after death.

What is the theme of "How It Happened" by Arthur Conan Doyle?

Sir Arthur Conan Doyle was a convinced believer in spiritualism, which is based on the belief that human souls live on after death and that living persons can communicate with them, usually through "mediums." It is often overlooked that Doyle begins this story with the following brief explanation:

She was a writing medium. This what she wrote:--

Then the story immediately becomes a first-person narrative by a wealthy country gentleman who loses control of his car and after a harrowing drive down a winding road is killed in a smash-up in front of his home. The narrator is already dead when he is telling this story through the medium, but the reader does not fully realize this until the end, when the narrator meets his old friend Stanley who died during the Boer War.

"Stanley!" I cried, and the words seemed to choke my throat--"Stanley, you are dead."
He looked at me with the same old gentle, wistful smile.
"So are you," he answered.

The excitement of the wild ride down the hill makes the reader forget that the story is being narrated through a so-called "writing medium," so it will come as a shock to realize that the narrator is fully conscious but has joined the invisible ranks of the dead.

Doyle sincerely believed in life after death and in the possibility of communicating with the dead through spiritualist mediums. He lost his beloved son during World War I, and this experience had a lasting effect on his mind. The main theme of his short story "How It Happened" therefore would appear to be that human souls survive after death.

There are a few different possible themes to this (outside of the simple suspense of finding out what happens to the narrator).

First, you can argue that a theme is that little decisions made out of foolishness and overconfidence can (but don't always) have a huge price.  The narrator admits this at one point in the story.  As we read the story, we know that he has made a mistake.  He should have just let Perkins drive.  But we make mistakes like that all the time and don't get badly punished.  In a way, then, this is a story about the vagaries of luck and fate.  Two actions on our part, both equally stupid, could have very different effects due only to luck.

Another possible theme has to do with the nature of courage and duty.  In this view, Perkins is as important as the narrator in showing the theme.  Perkins, for example, offers to try to drive the car while his master attempts a jump to safety.   The narrator declines and offers Perkins the same chance.  He too declines.  They seem to feel a sense of duty to one another and they are both (one can argue) showing bravery in accepting that duty.  Perkins shows his feelings towards the end when his first words to the rescuers are a question about how his master is.

Either of these could be seen as the major theme of the story.