Quote 16
The old man uses his own determination to test the limits of that of the fish.
The old man’s determination pays off; while he cannot beat the fish in strength, he slowly wears him down instead.
Quote 17
That the old man speaks to himself illustrates the intensity of the situation; he has to verbally convince himself to endure.
Because the old man addresses his body, we see a difference in psychological determination, which is undying, and physical determination, which has its limits.
Quote 18
Because the old man speaks alternatively to himself and to the fish, it is at times unclear to which he is speaking. This suggests there is not a lot of difference between the old man and the fish,
Even after the fish is dead, the old man’s is determined to finish the job without rest.