The Old Man says, "A man can be destroyed but not defeated."
Santiago decides to take a great risk to prove to himself that he still possesses the skill of a great fisherman after not having caught a fish for eighty four days. Once he meets a much more powerful opponent, the marlin, he questions his ability to conquer this noble and beautiful fish. He is old. He is tired. He is in pain. He is weak. Many self doubts creep up.
This is when Santiago draws on his inner resources for strength and hope: he thinks about his hero, DiMaggio, who played great despite the agonizing pain of a bone spur; he remembers his youth glory days when he won an arm wrestling match and earned the title of El Champion; he dreams of the young lions on the beech, wishing to regain the strength and vitality of his youth; he even prays.
What does Santiago mean by saying a man can be destroyed but not defeated? He is talking about something human beings possess that makes us, hopefully, rise above the animal world--the indestructible human spirit, courage, and determination. After a super-human battle with the marlin, Santiago still has to fight off the despicable sharks that relentlessly try to destroy the results of his hard labor. Though he realizes that his efforts are futile, and they keep coming all night, Santiago continues to spear, stab, club and mercilessly kill them. He refuses to admit defeat.
Another inner conflict is Santiago's guilt for destroying the noble and powerful marlin. While Santiago did need to fish in order to live, he admits that he went too far into the marlin's territory, and, in essence, tricked him. He refers to the fish as his brother and shows great respect for his beauty and innocence. The fact that the sharks destroy the meat and have him return to shore with no more than a skeleton, makes Santiago feel very remorseful for going out too far just to soothe his own wounded ego. He destroyed a noble creature only for pride. What a waste.