Where does it state the internal conflict in the story "To Build a Fire"?
In "To Build A Fire," the main external conflict can be expressed as "man against nature" -- it is simply too cold (75 degrees below zero!) to travel alone. The internal conflict is the man's hubris and self doubt. The root of this conflict is expressed in the third paragraph:
The trouble with him was that he was without imagination. He was quick and alert in the things of life, but only in the things, and not in the significances. Fifty degrees below zero meant eighty-odd degrees of frost. Such fact impressed him as being cold and uncomfortable, and that was all....That there should be anything more to it than that was a thought that never entered his head.
The man, in other words, is not smart enough to know better. His lack of "imagination" is doubly damning -- not only is he incapable of imagining the sheer magnitude of the frozen waste in which he finds himself, he lacks the imaginative foresight to prevent problems from occurring, or to solve them once they happen. In this, London distinguishes him from the dog, who instinctively knows that travel is dangerous and who follows the man out of a (mistaken) faith in his ability to produce fire.
This internal conflict begins to take shape as the man slowly begins to realize the magnitude of his problem, and that the "old timer" back at the camp might have known better than he did about the nature of travelling in extreme cold. While at first he thought the old timer "womanish" for his caution, after his second fire is blotted out he realizes that "Perhaps the old-timer on Sulphur Creek was right. If he had only had a trail-mate he would have been in no danger now."