A demonstration must have as starting-points appropriate principles (NLP I.19, 106–7): true, first, immediate, prior to, better known than, and causes of the conclusion (NLP I.6, 34; DOS 503; 558). Of these conditions, to be true, first (or primitive), and immediate are the central ones: first because there is nothing prior to it in its kind—otherwise the source of scientific knowledge rests on an infinite series of demonstrations (NLP I.8, 43; I.11, 53); immediate because there is no medium bet