Throughout "To Build a Fire," London mainly uses tactile imagery to emphasize the effects of cold on the man's body: his face and hands grow numb in the cold air, and feeling returns in the form of pain and tingling when he beats his hands or stamps his feet. Of all the senses, touch is perhaps the most instinctual—while visual and auditory information enter the brain through specialized organs like the eyes and ears, tactile information can come from almost anywhere in the body. Touch is also the most reliable sense—while it is fairly easy to misread or mishear something, one is far less likely to "mis-feel."
But once the man begins to freeze to death, he finds himself no longer able to rely on his most basic sense:
The man looked down at his hands in order to locate them, and found them hanging on the ends of his arms. It struck him as curious that one should have to use his eyes in order to find out where his hands were.
London's imagery in this passage showcases how the man's senses have become jumbled, as he sees rather than feels his hands. No longer able to instinctually sense the location of his own limbs, the man must rely on his eyes instead, a task that becomes more difficult as his plight becomes more desperate, until the man feels entirely disconnected from his own body:
It struck him as curious that he could run at all on feet so frozen that he could not feel them when they struck the earth and took the weight of his body. He seemed to himself to skim along above the surface, and to have no connection with the earth.
Enlightenment thinkers in the 17th and 18th centuries valued reason and the evidence of the senses, a way of thinking that carried over into the Naturalist movement. Unlike some other Naturalist authors, however, Jack London seems less confident in humans' ability to truly understand and control the natural world through rational thought. London's unusual use of tactile imagery in "To Build a Fire" shows how environmental forces can render the senses—and therefore scientific knowledge as a whole—unreliable.
When it comes to descriptions of the setting, "To Build a Fire" relies mainly on visual imagery, and the absence of other types of imagery serve to underscore the bleakness of the landscape and the man's isolation—there are no descriptions of sound, for example, because it is silent, and the man is alone. There is, however, one key moment in which London utilizes auditory imagery:
As he turned to go on, he spat speculatively. There was a sharp, explosive crackle that startled him. He spat again. And again, in the air, before it could fall to the snow, the spittle crackled.
London's vivid description of the sound that saliva makes when it freezes emphasizes the sheer cold of the setting. This moment is also important because it draws attention to London's argument regarding scientific and instinctual knowledge. By spitting into the cold air, the man is performing a sort of experiment to measure the temperature, and he draws conclusions based on his prior knowledge:
He knew that at fifty below spittle crackled on the snow, but this spittle had crackled in the air. Undoubtedly it was colder than fifty below—how much colder he did not know.
A few paragraphs later, the narrator clarifies that it is actually 75 degrees below zero and that the dog, despite its ignorance of things like thermometers, is instinctually aware of how cold it is. In this passage, London seems to reject Naturalism's emphasis on observation and the scientific method. The man has followed the scientific method to the letter, but his reasoning is still not as accurate as the dog's instinct.
