Throughout the story the narrator draws a contrast between what the man thinks and his perception of the conditions he finds himself in, and the dog's instinctual nature. The man rationalises everything and ignores common sense as illustrated in the following extract:
...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... 50 degrees below zero meant a bite of frost that hurt and that must be guarded against by the use of mittens, ear coverings, warm moccasins, and thick socks. 50 degrees below zero was to him nothing more than 50 degrees below zero. That it should be more important than that was a thought that never entered his head.
Conversely, the narrator says the following about the dog:
The animal was worried by the great cold. It knew that this was no time for traveling. Its own feeling was closer to the truth than the man’s judgment.
The contrast between the man's rationale and the dog's instinct makes it clear that its innate response is superior to the man's logic. This further implies that the risk cannot be thought away and that it would have been better if the man had responded to a natural urge to ensure survival.
The narrator further explains that the dog had no notion of temperature, but that it was intrinsically and keenly aware of the risks presented by extreme cold. As he states:
But the animal sensed the danger. Its fear made it question eagerly every movement of the man as if expecting him to go into camp or to seek shelter somewhere and build a fire. The dog had learned about fire, and it wanted fire. Otherwise, it would dig itself into the snow and find shelter from the cold air.
Unlike the man, the dog automatically knew about the dangers of ice forming on its paws. The man, although he had taken precautions of not landing in water, tended to ignore the urgent need to keep himself warm and delayed in making a fire. A further illustration of the animal's inherent awareness is illustrated in the following extract, where the dog, after falling through the ice and wetting its paws, found ice between its toes:
Then it lay down in the snow and began to bite out the ice that had formed between the toes. The animal knew enough to do this. To permit the ice to remain would mean sore feet. It did not know this. It merely obeyed the commands that arose from the deepest part of its being.
Once the man had made a fire and warmed himself, the dog was reluctant to leave when the man wanted to. The narrator explores the dog's perception:
This man did not know cold. Possibly none of his ancestors had known cold, real cold. But the dog knew and all of its family knew. And it knew that it was not good to walk outside in such fearful cold. It was the time to lie in a hole in the snow and to wait for this awful cold to stop.
The man is later forced to start another fire since he had mistakenly stepped through ice and wet his legs up to his knees. He had to dry his feet, socks and legs, otherwise he would freeze:
He worked slowly and carefully, realizing his danger. Gradually, as the flame grew stronger, he increased the size of the sticks with which he fed it.
Unfortunately, he had made the fire at an inappropriate place and it was killed by snow falling from a tree. The man realised that he needed to desperately start a new fire. He discovered, to his utter dismay, that he could not do so since he had been slowly freezing and his hands were numb. The attempts he mad all resulted in failure. The man then realised that he could only save himself by using the dog's warmth. He would have to kill it, though.
He spoke to the dog, calling it to him. But in his voice was a strange note of fear that frightened the animal. It had never known the man to speak in such a tone before. Something was wrong and it sensed danger. It knew not what danger, but somewhere in its brain arose a fear of the man. It flattened its ears at the sound of the man’s voice; its uneasy movements and the liftings of its feet became more noticeable. But it would not come to the man. He got down on his hands and knees and went toward the dog. But this unusual position again excited fear and the animal moved away.
We once again read in this extract how the dog's instinct is what saves it. It refuses to listen to the man's instructions for it senses that the man wishes to harm it. This is the man's final desperate attempt to save himself. In the end, he realizes that there is nothing he can do and gives in to death. The dog survives and the ending suggests that it may reach the end of its journey and find comfort with other humans.
Then it turned and ran along the trail toward the camp it knew, where there were the other food providers and fire providers.
Instinct is such an important concept in Jack London's "To Build a Fire" because it is the difference between life and death under the extreme conditions of the Yukon Territory.
The man in Jack London's story represents any person who finds himself unprepared for a new environment. While the word instinct is more often applied to animals than humans, Jack London perceived man in the Darwinian sense. That is, man's behavior was determined by heredity and environment.
In London's story, the man is unprepared for his environment; furthermore, he does not listen to the warnings of the older man from Sulfur Creek when told how cold it could become in the country. Because he "lacks imagination" he pays no attention and, instead, allows his greed to prod him onward to Henderson Creek where others await him to begin work on a claim. Accompanying him is a dog, whose instincts tell him that it is too cold to be out in the weather; consequently, there is "no keen intimacy between the dog and the man" as is often the case. Furthermore, while the dog knows that man usually provides fire, when the man fails at making a fire and dies, the dog catches the scent of death and instinctively knows to back away and return to the camp where he is sure to find "the other food providers and fire providers."