As Spekkio told Crono and his party at the End of Time, the physical world is made of four basic elements -- Fire, Water, Lightning, and Shadow. The balance of these four basic materials compose everything in the universe.
Magic is divided into 4 types: Lightning, Fire, Water, & Shadow. Not just magic, but EVERYTHING is based on the balance of these 4 powers.
While any force can manipulate the physical world, just as one can smash a chair or sculpt clay, not everyone can directly manipulate these four basic forces of the universe. The ability to do so is called magic. Magic is not some kind of power force found in things such as Lavos and the Sun Stone; rather, it is the direct control of Fire, Water, Lightning and Shadow. Due to its unique nature, Shadow is somewhat composed of the other elements combined. It can be made from only Fire and Water (as in Antipode), or from all three other elements.
To remove religious references from Chrono Trigger, Heaven was replaced by Lightning. The common compromise for both these terms in discussion is simply Light.
- Even though Ayla can't use magic, she has a small percentage of resistance against Fire magic (each character his a small resistance against something). This suggests that if Ayla had the genetics for magic, she'd be a Fire innate.
- Robo (partial)
- Schala (theorized), and probably other members of Zeal's royal family and the Gurus