Read the passage given below and pick the option that best fits the question that follows:
It happened one day, about noon, going towards my boat, I was exceedingly surprised 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 listened, I looked round me, I could hear nothing, nor see anything. 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 does the author convince himself that the foot-print is a real one?