Bede the Venerable — as excerpted in the Catena Aurea on Mark 9:43-50

“(ubi sup.) A friend is called a foot, on account of its service in going about for us, since he is as it were ready for our use. It goes on: And if thine eye offend thee, pluck it out: it is better for thee to enter into the kingdom of God with one eye, than having two eyes to be cast into hell fire; where their worm dieth not, and the fire is not quenched. A friend who is useful, and anxious, and sharp in perception, is called an eye.”