Full Idea
There is a great difference between a mind and a body, in that a body, by its very nature, is always divisible, but the mind is utterly indivisible.
Gist of Idea
The mind is utterly indivisible
Source
René Descartes (Meditations [1641], §6.85)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.164
A Reaction
This strikes me as being simply false. I don't just mean that surgeons can split the mind in half. We should think of the mind as a team of conscious and non-conscious processes, which are held together by a self in normal healthy people. Selves change.