What is "res extensa"

Res extensa is one of the three substances described by René Descartes in his Cartesian ontology (often referred to as "radical dualism"), alongside res cogitans and God. Translated from Latin, "res extensa" means "extended thing". Descartes often translated it as "corporeal substance". In Descartes' substance–attribute–mode ontology, extension is the primary attribute of corporeal substance.

Descartes describes a piece of wax in the Second Meditation (see Wax argument). A solid piece of wax has certain sensory qualities. However, when the wax is melted, it loses every single apparent quality it had in its solid form. Still, Descartes recognizes in the melted substance the idea of wax.