London writes, “He was quick and alert in the things of life, but only in the things, and not in the significances” (498). What does this tell us about the man? What is his character like?

London's description indicates that the man's mental functions are limited--not in the sense that he's lacking intelligence, but that his mind is focused narrowly upon superficial and mechanical things, the "things of life." The "significances," those things the man doesn't grasp, are the key to survival. It's as if he has a sort of tunnel vision that applies not merely to his eyesight, but to everything else. His mind does not have the capacity to draw conclusions. He's learned the essential facts about conditions in the Yukon--for instance, that if one's spit freezes in the air, before it reaches the ground, the temperature must be less than 50 degrees below zero Fahrenheit. But he's unable to conclude from this fact that he is in extreme danger and that it's likely he'll die before getting back to camp. Ordinary common sense eludes him, above all in his failure to grasp that traveling alone under these conditions is absurdly dangerous.
The man understands things he has been taught, such as how to build a fire by slowly feeding it with larger and larger twigs until it gets going fully. But he's unable to conclude that to build the fire underneath a tree is wrong, because the heat from the fire may (and does) cause the snow from the overhanging boughs to melt and then collapse upon the fire and extinguish it. The dog has a better understanding of the overall situation, in its instinctive way, than the man does. The man fails to conclude that the dog's survival instinct will cancel out his (the man's) usual authority over it. Though the dog is normally fearful and obedient, its will to live overrides this submissiveness and prevents the man from killing it as he attempts to do. This is the ultimate, and fatal, result of the man's being alert "in the things of life, but not in their significances."