> How do you keep the weeds out of your garden? In real life, you pull the weeds or specifically target them with pesticide. A wall is too granular of a filter

> How do you keep the weeds out of your garden? In real life, you pull the weeds or specifically target them with pesticide. A wall is too granular of a filter to keep weeds out. In software, the most successful filters -- such as e-mail spam filters --- specifically target malfeasors. A "wall" type of solution to e-mail would instead only trust e-mails from specific domains or recast the problem as "Facebook messages", eliminating interoperability. Walls don't exist to keep out weeds, they exist to keep out neighbors or poachers -- aka, competitors.