This article argues that the peculiarly ‘common law tradition’ separation of common law and equity had at its origins a principled basis in the concept of ‘conscience’. But ‘conscience’ here did not mean primarily either the modern lay idea, or the ‘conscience’ of Christopher St German's exposition. Rather, it referred to the judge's, and the defendant's, private knowledge of facts which could not be proved at common law because of medieval common law conceptions of documentary evidence and of trial by jury. The concept of a jurisdiction peculiarly concerned with this issue allowed the ‘English bill’ procedure to be held back to a limited subject area rather than—as in Scotland and the Netherlands—overwhelming the old legal system. By the later 17th century, however, the concept of conscience had lost its specific content, leaving behind the problem, still with us, of justifying the separation of ‘equity’.