While we are ignorant whether a man were aggressor or not, how can we determine whether the person who killed him be criminal or innocent?
 David Hume, An Enquiry Concerning the Principles of Morals (1751). copy citation

Context

“All the circumstances of the case are supposed to be laid before us, ere we can fix any sentence of blame or approbation. If any material circumstance be yet unknown or doubtful, we must first employ our inquiry or intellectual faculties to assure us of it; and must suspend for a time all moral decision or sentiment. While we are ignorant whether a man were aggressor or not, how can we determine whether the person who killed him be criminal or innocent? But after every circumstance, every relation is known, the understanding has no further room to operate, nor any object on which it could employ itself. The approbation or blame which then ensues, cannot be the work of the judgement, but of the heart;” source