Which detail from "To Build a Fire" best supports the idea that survival was not guaranteed along the actual Yukon trail?

The admonishment that no man should travel alone when the temperature is 60 degrees below zero from the old timers was shown to be true in this man's case since he fell through the ice and wet his feet and legs and depended on lighting and keeping a fire going himself. It turned out that nature in these conditions can be totally unforgiving.