After pondering this a bit, I'm going with Germ Theory.
The discovery, proof and general acceptance that no, your leg is not rotting off because of a magic spell / offending the leg god / not sacrficing your goat.
The correlation of cause & effect;
1- Made them look for causes for every condition.
2- Discover cures for as many conditions as they can.
3- Probably most important, establish methods of prevention.
