To obtain a conviction the state has to prove you guilty “beyond a reasonable doubt.” That means that it is not enough to make the judge or jury think you may have committed the crime, or even that you probably did. Rather, the state has to prove that you committed the crime with evidence so strong that the members of the jury are satisfied that there is no remaining reasonable doubt about the Commonwealth's version of the story. As the defendant you don't have to prove anything. As the defendant you cannot be forced to testify, and the burden of proving it's case remains with the state from the beginning to the end.