(Exodus 23:4) commands: "If you encounter the ox of your foe, or his donkey, straying, return shall you return it to him." The Mekhilta asks: does "encounter" mean literal physical contact? Must you bump into the animal to trigger the obligation?
(Exodus 23:5) uses a different word: "If you see" the animal of your enemy struggling under its burden. "Seeing" implies distance — you can see something from far away. Perhaps, then, the obligation activates even when you spot the lost animal from a mil away.
The Mekhilta reconciles the two verses. "Encounter" would be too narrow — physical contact is not required. "See" would be too broad — spotting something at extreme distance should not create an obligation. The sages estimated the actual distance as one-seventh and a half of a mil, which equals a ris — a specific measure of distance that falls between touching and sighting.
This practical ruling converted a vague biblical obligation into a precise legal standard. You are obligated to return a lost animal when you are within a ris of it — close enough to clearly identify it as lost and to reasonably assist, but not so close that you must literally stumble over it. The Mekhilta found the middle ground between two verses and expressed it as a measurable distance, transforming ethical aspiration into enforceable law.