Robinson was shocked and frightened one day when he saw a man's footprint in the sand along the shore. He ran to his home (which he now called his castle), stayed there all day, and was unable to sleep all night. At first he thought the footprint was from the devil, but then reasoned that it was likely from a savage who had ventured to the island by canoe.
Whereas Robinson would have previously wished for someone to come and rescue him, now he has come to appreciate his solitude so much that he reacts to the possibility of someone else on the island with only fear. At the same time, the footprint represents the return of the unknown to his life, when he had so civilized the island that it had become thoroughly known to him.
Robinson was thankful that he was not seen by these savages, but worried that they would see his enclosure with the goats, and his crops. His fear took away all his "former confidence in God." Robinson calls life a strange "chequer-work of Providence," and notes how strange it is that, all alone without any other humans, he was so terrified by this prospect of another human being on the island.
Robinson automatically assumes that any native people he might encounter are savages. His fear causes his strong faith in God to waver. In retrospect, Robinson notes how strange his reaction to the possible presence of another person was.
Robinson continued to reflect on the situation and concluded that it was not his place to question the "providence of God," and that he should "resign" himself to God's will. He remembered the Bible verse, "Call upon me in the day of trouble, and I will deliver thee, and thou shalt glorify me," prayed, and regained his spirits.
Robinson's faith returns and his belief in God helps him through his fear. The idea of providence provides a comforting structure or purpose behind events like his finding the footprint.
After three days within his "castle," Robinson ventured out into the island again. He reasoned that the footprint could have been an old one of his, and convinced himself that there was nothing to worry about. However, when he returned to the footprint, he saw that it did not match the size of his foot.
Robinson stays safely insulated and isolated from the world within his "castle" for three days before finally venturing out and examining the footprint, a threat of intrusion and the unknown into his life of solitude.
Robinson says that fear often causes men to discount reason and describes how he planned to destroy his livestock enclosure and send his goats into the wild and destroy his crops and "country house," so that whoever else was on the island wouldn't see these signs of habitation.
Robinson continues to fear the unknown stranger who left behind the footprint, whom he assumes would be hostile. He generalizes from his own experience in order to give the reader advice about fear clouding one's reason.
Robinson admits that he did not cry out to God in his distress instead of relying on "his providence." He thought that savages perhaps often went to his island by boat, so he decided to expand the fortifications around his dwelling, adding another outer wall with holes through which he could fire his guns. Scared of losing all his goats if attacked, he scouted out another place where he could build an enclosure for some of the goats, so that they would not all be in the same place.
Robinson prepares to defend his isolated home against any savages (though he has yet to encounter any). His fear again causes him to momentarily forget his trust in divine providence.