“I don’t know why dogs always go for postmen, I’m sure,” continued our guide.
“It’s a matter of reasoning,” said Poirot. “The dog, he argues from reason. He is intelligent, he makes deductions according to his point of view. There are people who may enter a house and there are people who may not–that a dog soon learns. Eh bien, who is the person who most persistently tries to gain admission, rattling on the door twice or three time a day–and who is never by any chance admitted? The postman. Clearly, then, an undesirable guest from the point of view of the master of the house. He is always sent about his business, but he persistently returns and tries again. Then a dog’s duty is clear, to aid in driving this undesirable man away, and to bite him if possible. A most reasonable proceeding.”
– Agatha Christie, Dumb Witness