ABSTRACT
In the Discourse on Metaphysics Leibniz put forward his famous complete-concept definition of substance. Sometimes this definition is glossed as stating that a substance is an entity with a concept so complete that it contains all its predicates, and it is thought that it follows directly from Leibniz’s theory of truth. Now, an adequate definition of substance should not apply to accidents. But, as I shall point out, if Leibniz’s theory of truth is correct then an accident is an entity with a concept so complete that it contains all its predicates.
The aim of this paper is to clarify Leibniz’s notion of substance in the Discourse with a view to explaining how that definition successfully distinguishes between substances and accidents. I shall argue that there is a sense in which accidents have complete concepts and a sense in which they don’t, while there is no sense in which substances do not have complete concepts. Key to my argument is the idea that according to Leibniz a substance is its own subject. Although that idea might seem odd, I present evidence that Leibniz accepted it. Although the paper concentrates on the Discourse, discussion of a note on one of Leibniz’s letters to Arnauld will be important.