Arguably, Santiago has been fairly clear-headed throughout the days that he has been at sea. Alone on the vast ocean, the experienced Santiago talks to himself in order to remain alert, he philosophizes about the stars, moon, and sun.
After the moon comes up on the second night, the great fish continues to pull Santiago's boat eastward; finally, he leaps from the water, just as Santiago has dreamt of the porpoises. As the marlin leaps and dives, Santiago's mind is sharp:
This is what we waited for, he thought. So now let us take it.
Make him pay for the line, he thought. Make him pay for it.
....The speed of the line was cutting his hands badly but he had always known this would happen...Yes, he thought. And now he has jumped more than a dozen times....He will start circling soon and then I must work on him.
Santiago's experienced and acutely analytical mind is alert constantly. When he becomes fatigued or has a cramp in his hand, the old fisherman thinks about how to deal with his disadvantages. At times, he encourages himself; for instance, as he looks at his damaged hand, he says, "It's not bad....And pain does not matter to a man."
When he believes that he is not entirely clear-headed, and when he feels hunger, the Spartan Santiago tells himself, "It is better to be light-headed than to lose your strength from nausea." So, he always is thinking and drawing upon his vast experience.