On a more serious note:
"Laws" of magic do exist, sort of, in the real world. Aleister Crowley developed some of these back in his day. Gygax even toyed with it in his "Dangerous Journeys" RPG.
Basically, the laws are along the lines of:
Law of Sympathy: Like affects Like. Voodoo dolls, for example, use the law of sympathy. A doll in the likeness of the person to be efected, peraps even a lock of hair, fingernails, etc. to "fuel" the magic.
Law of Opposition: Opposites Cancel. Fire defeat cold and visa versa. In a D&D context, a Fireball could be defeated (counterspelled) by a Coldball.
There are a bunch more, but it gets a little convoluted for my tastes. If I could have my druthers.