Crusoe sees that fatal on the island and realizes that (gasp!) he's not alone. He describes the scene as follows:
It happen'd one Day about Noon going towards my Boat, I was exceedingly surpriz'd with the Print of a Man's naked Foot on the Shore, which was very plain to be seen in the Sand: I stood like one Thunder-struck, or as if I had seen an Apparition; I listen'd, I look'd round me, I could hear nothing, nor see any Thing, I went up to a rising Ground to look farther, I went up the Shore and down the Shore, but it was all one, I could see no other Impression but that one, I went to it again to see if there were any more, and to observe if it might not be my Fancy; but there was no Room for that, for there was exactly the very Print of a Foot, Toes, Heel, and every Part of a Foot; how it came thither, I knew not, nor could in the least imagine. But after innumerable fluttering Thoughts, like a Man perfectly confus'd and out of my self, I came Home to my Fortification, not feeling, as we say, the Ground I went on, but terrify'd to the last Degree, looking behind me at every two or three Steps, mistaking every Bush and Tree, and fancying every Stump at a Distance to be a Man; (130)
Why might the footprint be terrifying to Crusoe?