cannot be found guilty...? of a crime because they lack evidence that they DID NOT commit it. For example, you can't be found guilty of theft if you cannot prove that you didn't steal something. Any ideas?
It's called the "presumption of innocence". Every person is presumed in law to be innocent of a crime that he's charged with, unless the government can prove that he's guilty.
You never have to prove you didn't do something - the government has to prove that you did.