What is the basic law of self-defense?

First, you should read the force laws for your state, county and city. But, in the meantime I found this quote in an article by Mr. Rory Miller that can give you a fundamental definition to get you started, i.e. "The basics of self-defense law, going all the way back to English Common Law, can be summed up: You may use the minimum force you believe is reasonable necessary to safely resolve the situation.

Be careful here. There are caveats to this statement and this is why is is imperative to anyone practicing martial arts or any means of self-protective measures of physical and emotional violence to know, understand and teach/practice with force law as a basis of said teaching and practicing.

Mr. Miller provides, see last post link to article, a bit of meaning behind such terms used above as "may," "minimum force," and "reasonably necessary." Do not assume the quote above is applicable to all laws for I feel those laws are complicated and convoluted when coupled with violence and all it entails. This is why there are "experts" in violence and force law - no one person necessarily knows both sides of this coin either.

I am not an expert, this is just an opinion and theory.