One theme in Hemingway's The Old Man at the Bridge is sense of duty. The old man the soldier meets at the bridge feels it is his duty to act as a shepherd, and watch over his flock. The old man believes that he must watch over the four doves, the cat, and the two goats in San Carlos. The artillery fire is why he and others of the town are evacuated.
The young soldier feels that it is his duty to carry out the orders of the evacuation to ensure overall success in the war. While he encounters the old man at the bridge and feels some empathy for him, the soldier does nothing to aide the old man. The old man is tired and old yet the soldier does nothing for him, opting to keep to his sense of duty to the military and his orders.
It is up to the reader to decide which man is the better person. The old man places the needs of other living things over his own well being and the need to save himself. While he does leave the living things behind, it is due to the fact of old age and the forced evacuation. He would have taken care of them otherwise. The old man represents nature and morality. His duty is to care for nature.
The soldier places the need of the man made and unnatural as priority. He offers a kind ear to the old man but does nothing physically to change the old man's situation. While the soldier has orders, he could have taken a moral point of view in the old man's dilemma. The young soldier represents man's neglect of nature and tendency toward war or violence.