"Danger of falling down every Precipice"

Swift is referring to the story of Thales, the philosopher of Miletus, who lived about 600 B.C. 

Staring at the stars while walking, he failed to notice a ditch and fell in. A bystander laughed and said, "Here is a man who would understand the motion of the stars, yet who cannot see what lies at his feet."

Thales is the prototype for the "absentminded professor".