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## Eidos

*Print version* ISSN 1692-8857

#### Abstract

MONNOYEUR BROITMAN, Franfoise. **The Indefinite within Descartes' Mathematical Physics**.* Eidos* [online]. 2013, n.19, pp.107-122.
ISSN 1692-8857.

Descartes' philosophy contains an intriguing notion of the infinite, a concept labeled by the philosopher as indefinite. Even though Descartes clearly defined this term on several occasions in the correspondence with his contemporaries, as well as in his Principles of Philosophy, numerous problems about its meaning have arisen over the years. Most commentators reject the view that the indefinite could mean a real thing and, instead, identify it with an Aristotelian potential infinite. In the first part of this article, I show why there is no numerical infinity in Cartesian mathematics, as such a concept would be inconsistent with the main fundamental attribute of numbers: to be comparable with each other. In the second part, I analyze the indefinite in the context of Descartes' mathematical physics. It is my contention that, even with no trace of infinite in his mathematics, Descartes does refer to an actual indefinite because of its application to the material world within the system of his physics. This fact underlines a discrepancy between his mathematics and physics of the infinite, but does not lead a difficulty in his mathematical physics. Thus, in Descartes' physics, the indefinite refers to an actual dimension of the world rather than to an Aristotelian mathematical potential infinity. In fact, Descartes establishes the reality and limitlessness of the extension of the cosmos and, by extension, the actual nature of his indefinite world. This indefinite has a physical dimension, even if it is not measurable.

**Keywords
:
***Descartes*; *Aristotle*; *Nicholas of Cusa*; *Galileo*; *potential and actual infinity*; *mathematical and physical infinity*; *mathematical physics*; *space*; *matter*; *extension*; *subtle matter*.