Full Idea
I can understand myself without the faculties of imagining and sensing, but not vice versa; I cannot understand them without me - a substance endowed with understanding.
Gist of Idea
I can exist without imagination and sensing, but they can't exist without me
Source
René Descartes (Meditations [1641], §6.78)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.157
A Reaction
I think this is a fundamental and important error on Descartes' part. The idea that understanding is possible without imagination (and even sensation) is wrong, and it leads to the misleading concept of 'pure' reason.