A mule will labor ten years willingly and patiently for you, for the privilege of kicking you once.

William Faulkner

A mule knows its limits. It is characteristic of the breed to have an inviolable commitment to self-preservation, which is often misinterpreted as stubbornness.

Susan Orlean, Riding High: Mules in the military, The New Yorker (February 15 & 22, 2010)