Abstract

As is well known, Leibniz criticises Descartes for not having shown that God (considered as ens perfectissimum ) is possible, and tries to fill this gap by proving God’s possibility on the basis of absolutely positive and simple perfections. For many readers, however, these perfections have appeared problematic or unintelligible. In my paper, I primarily want to show, that they can be made comprehensible by working out their foundations in Plato’s theory of ideas. On this basis, I want to explain, how this allows to understand the compossibility of perfections, essences or ideas (in the Leibnizian regio idearum ) and how such an understanding fits into his version of the ontological proof. For this purpose, the first part of my paper follows two closely related lines of investigation. On the one hand, I focus on Leibniz’s theory of ideas and try to explain in which way he combines his criticism of Descartes’s proof with a criticism of the Cartesian understanding of clear and distinct ideas. On the other hand, I discuss his transformation of Platonic ideas into essences leading from actually existing entities to real possibilities. In order to provide a foundation for this, I start with a glance at Descartes’s ontological proof and at his replies to Caterus and Mersenne concentrating on the problem of God’s possibility. The second part of my paper will work out Leibniz’s own version of the ontological proof and its Platonic background.