Robinson Crusoe thought that the print was a footprint because it had toes, heels, and all parts of a human foot.
Although Robinson Crusoe longs for human contact, he feels he is much safer without it for now. After having a sleepless night, he thinks that the footprint belongs to one of the savages of the mainland, who had wandered out to the sea in a small boat. He goes out to the sea-shore to measure it with his own footmark.