Full Idea
Notoriously, the Cartesian idea that matter is purely geometrical will not do, for it leaves no distinction between matter and empty volumes: a filler for these volumes is required.
Gist of Idea
Matter can't just be Descartes's geometry, because a filler of the spaces is needed
Source
comment on René Descartes (works [1643]) by Howard Robinson - Perception IX.3
Book Reference
Robinson,Howard: 'Perception' [Routledge 2001], p.219
A Reaction
Descartes thinks of matter as 'extension'. Descartes's error seems so obvious that it is a puzzle why he made it. He may have confused epistemology and ontology - all we can know of matter is its extension in space.