 The rationalist metaphysics of Leibniz (1646–1716) is also foundational for the Enlightenment, particularly the German Enlightenment (die Aufklärung), one prominent expression of which is the Leibnizian rationalist system of Christian Wolff (1679–1754). Leibniz articulates, and places at the head of metaphysics, the great rationalist principle, the principle of sufficient reason, which states that everything that exists has a sufficient reason for its existence. This principle exemplifies the characteristic conviction of the Enlightenment that the universe is thoroughly rationally intelligible. The question arises of how this principle itself can be known or grounded. Wolff attempts to derive it from the logical principle of non-contradiction (in his First Philosophy or Ontology, 1730). Criticism of this alleged derivation gives rise to the general question of how formal principles of logic can possibly serve to ground substantive knowledge of reality. Whereas Leibniz exerts his influence through scattered writings on various topics, some of which elaborate plans for a systematic metaphysics which are never executed by Leibniz himself, Wolff exerts his influence on the German Enlightenment through his development of a rationalist system of knowledge in which he attempts to demonstrate all the propositions of science from first principles, known a priori.