Leibniz’s Idealism and the Problem of Extension

Martin Lin (Rutgers - New Brunswick)

Abstract: What are, for Leibniz, bodies, matter, and corporeal substances? In this paper, I argue that there is a significant strand in this thinking which combines both an idealistic and a realistic view of the physical world according to which the only substances are mind-like, and yet matter and the bodies it constitutes are nonetheless real because they themselves are constituted by the mind-like substances. Although this interpretation enjoys significant textual support and holds considerable philosophical interest, it faces serious difficulties as well. One of these concerns the nature and status of extension. As experienced by us and as theorized by the natural sciences, the physical world and its constituents are spatially extended. But if the monads are mind-like, and therefore are unextended, how then can they constitute extended things? The standard answer to this question among scholars who support the substance idealist/matter realist interpretation is to claim that when we ascribe many properties to bodies, including extension, we do so erroneously. That is, Leibniz is committed to a kind of error theory with respect to extension and other physical properties. I argue in this paper that such an error theory would pose serious problems for Leibniz given some of his other central philosophical commitments. In particular, I will argue that his theory of universal expression and its role in sense perception and mental representation more generally require him to hold that all perception is veridical. If this is true, then any interpretation that imputes massive error to the perceptions of the monads is untenable. But this is not a reason to reject substance idealism and matter realism as an interpretation of Leibniz. Instead, I will argue that a proper understanding of Leibniz’s theory of ideas points the way to an interpretation on which extension can be veridically attributed to aggregates of unextended monads.

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