A man has been shipwrecked on an island and believes himself to be alone there. One day, as he is walking along the beach, he discovers footprints. How did they get there? His first assumption is that they are his own. It occurs to him, however, that he had not been near this spot for over a week, and that yesterday’s storm would have washed any footprints away. This objection is confirmed by making a footprint himself and comparing it with the one observed, and noticing that they differ markedly. The footprints being those of some one else, how did the man who made them get there? The first supposition is that he came in a boat. The idea of a small boat is dismissed because of the assumed great distance of this island from other land. Therefore the man must have come in a large vessel. But the footprints lead to a wet part of the sand and the tide is just going down. In this case they are very recent—made not more than a half hour ago. This being so the man who made them could not have had time to get back to any ship and sail out of sight. If he came in a ship it should be still in view. The discoverer of the footprints climbs a tree from which he can view the sea around the entire island. He can sight no vessel. The supposition or hypothesis that the unknown came in a ship is abandoned. Then the suggestion comes that the unknown has been on the island during the entire time that the shipwrecked man thought himself alone. This suggestion is tested in a manner similar to the others. . . . The example sums up roughly the general process of all thought, and brings out the motive and value of observation. Let us analyze it.