The theory of how it’s supposed to work

“In the investigation and estimate of criminatory evidence, there is an antecedent prima facie presumption in favor of the innocence of the party accused, grounded in reason and justice not less than in humanity, and recognized in the judicial practice of all civilized nations, which presumption must prevail until it be destroyed by such an overpowering amount of legal evidence of guilt as is calculated to produce the opposite belief.” William Wills, An Essay on the Principles of Circumstantial Evidence