To what extent is Ernest Hemingway’s novel “The Old Man and the Sea” a conflict between man and himself, more than man and nature?

One could easily argue that the novel is about man against himself because Santiago faces great internal conflict.

He is an old, weak, and not a very strong man (at least by the looks of him!). He has also not caught a fish in 84 days. That is almost 3 months and a very long time when that is your livelihood. Imagine going to work for three months straight and being told you weren't good...

One could easily argue that the novel is about man against himself because Santiago faces great internal conflict.

He is an old, weak, and not a very strong man (at least by the looks of him!). He has also not caught a fish in 84 days. That is almost 3 months and a very long time when that is your livelihood. Imagine going to work for three months straight and being told you weren't good enough at your job to actually be paid. But he perseveres in spite of this challenge, and he decides to set out farther than usual. This is a risk, but one he feels he must take in order to reap the potential rewards that he needs to prove himself. He needs to prove himself not just to others, but also to himself.

As Santiago struggles with the marlin, and then with all of the other sea creatures out to take the marlin from him, he pulls on an internal strength he may not have known he still had. He finds mental and physical strength that is beyond what anyone would have expected from him. 

In winning the battle, so to speak, with the marlin and actually living to tell about it, he has proven he is still strong and capable. That he loses the marlin is unimportant because in battling the marlin and the other fish and sharks, he has actually also battled his own weaker, less capable self. He regains confidence: "You were born to be a fisherman as the fish was born to be a fish."