Context: The author has finished a harangue against the training given youths in college, especially the training in rhetoric, because they see and hear nothing of ordinary life there. Because of this belief, the author and his friend Ascyltos slip away and go adventuring. At the home of Trimalchio, "a very rich man, who has a clock and a uniformed trumpeter in his dining-room, to keep telling him how much of his life is lost and gone," they were to spend the day. They accompany Agamemnon, a teacher of rhetoric, and Menelaus, Agamemnon's assistant, to Trimalchio's door. The proverb about the dog has been found with a picture of a dog on a mosaic floor in Pompeii. Petronius thus describes his experience:
. . . Just at the entrance stood a porter in green clothes, with a cherry-coloured belt, shelling peas in a silver dish. A golden cage hung in the doorway, and a spotted magpie in it greeted visitors. I was gazing at all this, when I nearly fell backwards and broke my leg. For on the left hand as you went in, not far from the porter's office, a great dog on a chain was painted on the wall, and over him was written in large letters "Beware of the dog." My friends laughed at me, But I plucked up courage and went on to examine the whole wall. It had a picture of a slave-market on it, with the persons' names . . .