Full Idea
Nothing can be in me of which I am entirely unaware.
Gist of Idea
I can't be unaware of anything which is in me
Source
René Descartes (Reply to First Objections [1641]), quoted by Robert Pasnau - Metaphysical Themes 1274-1671 08.4
Book Reference
Pasnau,Robert: 'Metaphysical Themes 1274-1671' [OUP 2011], p.150
A Reaction
This I take to be a place where Descartes is utterly and catastrophically wrong. Until you grasp the utter falseness of this thought, the possibility of you (dear reader) understanding human beings is zero. Here 'I' obviously means his mind.