Principia
...the principia of any given discipline must be identified as a principium essendi, literally a "principle of being" or essential foundation—and a principium cognoscendi, a "principle of knowing" or cognitive foundation: the former is necessary for the existence of the discipline, the latter for knowledge of it.
Richard A. Muller, Post-Reformation Reformed Dogmatics: The Rise and Development of Reformed Orthodoxy; Volume 1: Prolegomena to Theology, 2nd ed. (Grand Rapids, MI: Baker Academic, 2003), 431.