Descartes believed the extended world did not terminate in a boundary: but why? After elucidating Descartes’s position in §1, suggesting his conception of the indefinite extension of the universe should be understood as actual but syncategorematic, we turn in §2 to his argument: any postulation of an outermost surface for the world will be self-defeating, because merely contemplating such a boundary will lead us to recognise the existence of further extension beyond it. In §3, we identify the fundamental assumption underlying this argument by comparing Descartes’s and Malebranche’s respective conceptions of the ontological status of modes of extension.
