Hemingway uses the allusion of 40 days to reference how long Santiago has been at sea. The allusion of "40" is seen in many works of fiction and in religious texts. In Abrahamic faiths, 40 is used in a number of references. For example, the ancient Israelites spent 40 years in the desert before they reached Israel, and Jesus Christ spent 40 days in the desert battling the temptations of Satan. So the use of 40 turns up as a time reference to describe how man experiences the onslaught of nature, whether it is heat of the desert or empty ocean.
"The Old Man and the Sea" illustrates an inherent struggle between Santiago and the marlin. This can be seen as a general antagonism between man and nature, a theme familiar in other popular ocean books such as "Moby Dick." Hemingway, however, presents Santiago as a man who does not want to be nature's antagonist. Hemingway does this, according to the University of Florida's Institute for Psychological Study of the Arts, by displaying every aspect of the sea as both friendly or inviting and violent or hostile. Santiago does not want to win a fight against nature by capturing a fish; he merely wants to capture a fish in and of itself.
A reviewer at Webster College counters the prevailing thinking of man vs. nature in "The Old Man and the Sea" by contending that Santiago is only proving himself as a man in nature. Santiago goes out to sea to capture this marlin, only to have the sea and its animals, such as sharks, attack him. When Santiago finally gets the marlin, sharks that are following Santiago attack his boat, leading the marlin carcass to fall back to the sea for the shark's meal. Instead of feeling despair, Santiago is proud of what he accomplished. He does not need the marlin as vindication that he was at sea for more than 80 days; he only needs to know himself that he conquered his fears and limitations by experiencing the harshness of nature.
"The Old Man and the Sea" includes a reference about how much respect Santiago gives nature. Santiago comes close to saying that he would rather be a beast in the bottom of the sea than a man living in society. This reference is trying to convey a literary theme in which nature is given the reverence of purity or superiority over society. This is elaborated in Santiago's journey to the sea. Not only does he not want to be an antagonist to nature, but he wants to be an equal to it. As the Institute or Psychological Study of the Arts at the University of Florida analyzes it, however, Santiago became too close to nature, almost disrespecting it. Similar to a Greek tragedy, the hero ventures too close to the gods in the name of pride. Because of this, Santiago loses the marlin and retains his respect for nature.