The moral lesson in Jack London's short story "To Build a Fire" is that people should not think they are more powerful than nature. In addition, people should listen to others who have more experience than they do. London writes of the protagonist of the story, "The trouble with him was that he was not able to imagine." Though the man is new to the Yukon Territory, he ventures out on a sunless day while thinking that he can survive the cold. Though it is fifty degrees below zero, he has no real conception of what that means to him, his body, and his safety. As London writes, "It did not lead him to consider his weaknesses as a creature affected by temperature. Nor did he think about man’s general weakness, able to live only within narrow limits of heat and cold." The man is limited by his previous experience, so he thinks, without any evidence or reason, that he can outwit the cold, though people have warned him that he can't. In reality, the temperature is even colder than 50 below zero, and as the man dies, he thinks, “You were right, old fellow. You were right" about the old man in Sulphur Creek who had warned him not to venture out. In the end, the man realizes that nature is more powerful than he is and that he should've listened to people with more experience in the region. 

The moral lesson implicit in Jack London's short story "To Build a Fire" is a stern and cheerless one. It might be expressed as follows. Man is alone in a pitiless, godless universe. He cannot look to any supernatural power for any kind of assistance. The best thing that humans can do because of their mutual human predicament is to cooperate and work together to make existence as comfortable and secure as is possible. But in the long run we are all doomed to the same extinction that occurred to the unnamed protagonist of London's story. Jack London was a socialist, and his story has a socialistic as well as an atheistic theme. London might also be called an existentialist, but he lived long before that term was coined by the French philosopher Jean-Paul Sartre.

The story calls to mind "The Open Boat" by Stephen Crane, as well as the essay by Sigmund Freud titled "The Future of an Illusion." Crane and London both resemble the pessimistic author Ambrose Bierce, whose best-known story, accessible in e-notes, is "An Occurrence at Owl Creek Bridge." The same theme can be seen in a number of Ernest Hemingway's stories, most notably in "A Clean, Well-Lighted Place." Jack London's stark, fatalistic view of life might well be summarized by quoting this very short poem of his contemporary Stephen Crane.

A man said to the universe:
“Sir, I exist!”
“However,” replied the universe,
“The fact has not created in me
A sense of obligation.”