 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 between the subject and predicate (NLP 6, 38). Whereas to be true, first, and immediate are the qualifications that concern what the principles are in themselves (and thus the ones which belong to their definition), the others—for example, to be better known—qualify the principles with respect to the conclusion (NLP I.8, 44; I.10, 49).