"You can only interpret the law if you don't know what it is" is a peculiar idea.
Why should there be a law against leaving things laying around?
How about buried land mines? Or razor blades in a daycare? Or a handful of nails on the road?
The guy should have watched where he was going.
You have come up with an exception to the general rule that "if you hurt somebody, you pay them back". Excellent. That's common law, and it's what judges do.
We don't need law schools and lawyers to have common sense applied in a civil case with a jury of peers and a reasonably intelligent judge.
Because sometimes people disagree on what common sense dictates. In that case, society defers to "what did we do last time?", and that's where law comes from.
In addition, some disputes are too complicated to have common sense applied. Homespun wisdom doesn't help much if the dispute is, say, between two steel companies over provisions in a asset sale and liability assumption agreement.
Correction: Because sometimes people and Sophists disagree on what common sense dictates.