One central claim that is common to the systems of Du Châtelet, Wolff, and Leibniz is that the extended, continuous, divisible bodies we encounter in the visible world are constituted by extensionless, discrete, indivisible, insensible entities. This essay focuses on Du Châtelet’s version of this general claim and examines how it fares against the objections raised by Euler in his Letters to a German Princess. Euler sees two main faults with the theories of those who posit simple unextended beings, namely (1) that they confuse physical and metaphysical divisibility and (2) that they introduce an incoherent distinction between two types of extensionExtension. Sections 8.2–8.4 of this essay focus on Du Châtelet’s answer to the former objection, which consists in a hierarchy of indivisibles. Section 8.5 comes back to Euler’s second point, which is answered by Du Châtelet’s doctrine of extension as a real phenomenonPhenomenon. On the whole, the core of their disagreement is that for Euler, extensionExtension is an irreducible property, while Du Châtelet holds (with Wolff) that it can be further explained. Whereas the previous sections focus on the content of Du Châtelet’s account of the derivation of extension from unextended simples, Sect. 8.6 turns to the question of its justification. Three separate ways of reading this justification are suggested, of which one that grounds the explanation in a psychological fact is both the most convincing and fits best with the text of the Institutions.