Leibnitz's metaphysical speculations are scattered through a wide variety of writings, many of which are letters to his contemporaries. These Professor Erdmann has incorporated in his edition of the Philosophical Works. Beside these we may mention, as particularly deserving of notice, the "Meditationes de Cognitione, Veritate et Ideis", the "Systeme Nouveau de la Nature", "De Primæ Philosophiæ Emendatione et de Notione Substantiæ", "Reflexions sur l'Essai de l'Entendement humain", "De Rerum Originatione Radicali", "De ipsa Natura", "Considerations sur la Doctrine d'un Esprit universel", "Nouveaux Essais sur l'Entendement humain", "Considerations sur le Principe de Vie". To these we must add the "Théodicée" (though more theological than metaphysical) and the "Monadologie", the most compact philosophical treatise of modern time. It is worthy of note, that, writing in the desultory, fragmentary, and accidental way he did, he not only wrote with unexampled clearness on matters the most abstruse, but never, that we are aware, in all the variety of his communications, extending over so many years, contradicted himself. No philosopher is more intelligible, none more consequent. In philosophy, Leibnitz was a _Realist_. We use that term in the modern, not in the scholastic sense. In the scholastic sense, as we have seen, he was not a Realist, but, from childhood up, a Nominalist. But the Realism of the schools has less affinity with the Realism than with the Idealism of the present day. His opinions must be studied in connection with those of his contemporaries. Des Cartes, Spinoza, Locke, and Leibnitz, the four most distinguished philosophers of the seventeenth century, represent four widely different and cardinal tendencies in philosophy: Dualism, Idealism, Sensualism, and Realism. Des Cartes perceived the incompatibility of the two primary qualities of being, thought and extension, as attributes of one and the same (created) substance. He therefore postulated two (created) substances,--one characterized by thought without extension, the other by extension without thought. These two are so alien and so incongruous, that neither can influence the other, or determine the other, or any way relate with the other, except by direct mediation of Deity. (The doctrine of Occasional Causes.) This is Dualism,-- that sharp and rigorous antithesis of mind and matter, which Des Cartes, if he did not originate it, was the first to develop into philosophic significance, and which ever since has been the prevailing ontology of the Western world. So deeply has the thought of that master mind inwrought itself into the very consciousness of humanity!