After all these years, on an uneventful day, without warning, Crusoe "was exceedingly surprised with the print of a man's naked foot on the shore." He "stood there like one thunderstruck, or as if I had seen an apparition." Confused in his thoughts, he retreated like an animal to his fortification. He could not sleep the entire night because he was so beset by apprehensions. In his confusion, he thought that it was the Devil attempting to trick him in some manner. He dismissed this idea and thought that, instead, it might be a savage from the mainland. He feared that his boat would be found and, thus, his presence on the island would be known. Suddenly, all of his confidence in God left him.
In this uncomfortable state of mind, he finally turned to the Bible for comfort and began to be more rational in his thinking. It occurred to him that the footprint that he saw might merely be one of his own. Even though his mind was not reconciled for weeks or months, yet on the third day of his hiding, the goats needed milking and he had to take courage to go out and tend to them.
Later, becoming more bold, he went down to the shore to measure his foot against the print. He discovered that the footprint was much larger then his own foot. His first thought was to destroy all of his enclosures that he had built in order to prevent being found out that he was on the island. But after considering, he realized that he had not seen anyone in the fifteen years that he had been there, and if anyone had accidentally landed, he had probably gone off again quickly.
He fortified his fortifications with another wall so that now he had a double wall. Concerned with his herd of goats, he resolved to preserve them in a different location. Finding a piece of ground in the middle of a thick woods, he fenced it in to secure his goats. He had still seen no human being, and he had spent two years in his uneasiness. He constantly prayed to God to protect him from danger.
Seeking another part of the island to put more goats, he spied a boat upon the sea. Seeing the boat overturned, his thoughts once again turned toward savages and cannibals. Appropriately enough, as he came down the hill to the seashore, he was horrified to find "the shore spread with skulls, hands, feet, and other bones of human bodies." Crusoe saw a circle "where it is supposed the savage wretches had sat down to their inhuman feastings upon the bodies of their fellow creatures."
Horrified and sickened by this sight, he rushed back to his habitation. Crying, he thanked God that he had never reached this level of degeneracy. It was his hope, having been there almost eighteen years, that he could be there another eighteen years without being discovered by such savages and wretches.
For two years, he ventured no farther than his fortifications, and never fired his gun for fear of detection. But after two years, and after thanking God for his preservation, he turned his thoughts to the possibility of brewing some beer out of some barley which he had grown. Finding no substitute for hops or yeast, he found it was impossible to brew any beer.
Crusoe began to entertain thoughts of how he could execute divine retribution against the savages in case they brought another victim to the island to devour and, thus, save the victim. He entertained several ideas about how he could accomplish this and decided to hide himself in a thicket of trees and ambush the savages.
After describing to us how he armed himself for this attempt, he made a daily foray to the top of the hill to look for boats coming from the sea. As the novelty of this idea wore off, he began to wonder at his authority to determine their fate or to judge their practices. If God suffered them to go unpunished, it could be that they saw no crime in what they did, it being their custom. After consideration, he saw himself in the wrong and decided only to prevent, if necessary, their bloody business and not to attack without provocation.
From a practical point of view, however, he realized that unless he killed everyone who came on shore, he could never be certain of his safety. Presently, he decided to go about his affairs and to conceal himself from them, leaving them to the justice of God: "I gave most humble thanks upon my knees to God that had thus delivered me from blood-guiltiness."
The discovery of the footprint in the sand is one of the most famous episodes in all of literature. The general public, however, usually thinks that it is Friday's footprint when, in reality, Friday does not appear for years later. It is never made clear whose footprint it really is, but in the view of later events, we can assume that the footprint belonged to one of the savages of whom Friday was a member. The discovery of the footprint prepares the way for the later appearance of the savages upon the shore and causes Crusoe to be more alert and cautious.
After the discovery of the footprint, two years elapsed before Crusoe had another frightening experience — that is, the discovery of the bones. This event causes Crusoe to contemplate the customs of cannibalism and what his duty should be if he were confronted with the cannibals. He dreams up various plots to kill the savages, yet he soon realizes this possibility: "what authority have I to be their executioner?" Despite this philosophical insight, however, it never enters Crusoe's mind as to the justice of killing many, many people in order to save one unknown savage.