For much of the century before Jack London started writing, Americans often wrote about how a return to nature would allow us to reach some sort of transcendent state or bliss (we're looking at you, Henry David Thoreau). London reacted to this tendency through a style known today as "literary naturalism," which depicted nature as a brutal force that was completely indifferent to humanity's existence or accomplishments. That's not to say that nature's an evil force in London's eyes. It just doesn't care one way or the other if humans are happy, or self-actualized, or, well alive. In "To Build a Fire," London plays this note constantly in his descriptions of the vast, brutal quality of the Yukon landscape, and the indifferent survivalism of the dog, who also couldn't care less if the guy lives or dies, as long as he can get his four paws near a fire.
In "To Build a Fire," Jack London shows us that nature's true value lies in the fact that it does not care about humanity.
Whether he has imagination or not, the man's thoughts mean nothing in the face of the vast and cold Yukon.