text: "a person is innocent unless proven guilty of a crime" is this argumentum ad ignorantiam?   no; the failure of a court system to convict a person does not logically imply that a person is truly innocent of the crime.  by the same token, the success of a court to convice someone does not logically imply that a person is truly guilty of the crime.  the courts do not seek to fully and logically prove things, only to establish them beyond reasonable doubt.  so, if the system is reasonably sure a person is guilty, they are punished.  if the system is reasonably sure a person is innocent, they are not punished.  the statement "innocent until proven guilty" simply defines the algorithm the government will follow in the case that it is not possible to reasonably determine whether a person is guilty or innocent.  the law isn't about strict logic as no empirical science can be, but is rather about fair and effective implementation of the law.