PART I. On the Idea of a Universal Language. Some Systems of Classifications of Universal Languages. A Proposal. Original Mythology and Its Presuppositions. Characterization of Adamic Language. Glossolalias. J. Böhme, Natursprache. Leibniz, Böhme and the Idea of Adamic Language. Emergence of Imaginary Languages, their Characterization. Imaginary Languages, Linguistic Theories. Decline in the Prestige of Latin and Emergence of Natural Languages. Future of International Languages. Mathesis Universalis. Specificity of Philosophical Projects. Bacon, Comenius, Urquart, Ward, Dalgarno, Wilkins. Function of a Universal Language According to Leibniz. Combinatory Predecessors: Llull, Kircher. Leibniz's Critiques. PART II. Symbolism in Leibniz: Cartesian Objection. Leibniz's Reply. Intuition and Symbolism. Mathematics as Model. Auxiliary or Constitutive Status of the Sign. Divine Thinking. Intuitive Knowledge. PART III. Diversity and Complementarity. Leibniz and German Language. Dissertatio de stylo Nizolii. Nouveaux Essais. Unvorgreiffliche Gedancken. Language Analysis. Rational Grammar. Latin as Basic Language. Rational or Philosophical Grammar. Reduction of Grammatical Categories. A Priori Projects. Characteristica Universalis. Operativity of the Sign. Representativity of the Sign and Its Models. Conclusion. Appendix: Critical Notes on Hobbes's Influence on the Leibnizian Project. Notes. Bibliography. Index of Names.