Full Idea
Descartes's claim that mind and body are separate because the first is necessary when thinking and the second isn't, is like arguing 'Superman's strength is indubitable; Clark Kent's strength is widely doubted; so Clark Kent is not Superman'.
Clarification
Should you be leading a sheltered life, you need to be aware that Clark Kent IS Superman
Gist of Idea
Superman's strength is indubitable, Clark Kent's is doubtful, so they are not the same?
Source
comment on René Descartes (Meditations [1641], p.156) by Keith T. Maslin - Introduction to the Philosophy of Mind 2.7.1
Book Reference
Maslin,Keith: 'An Introduction to the Philosophy of Mind' [Polity 2001], p.55
A Reaction
I've heard people defend Descartes on this, and Kripke is interesting on the subject, but Descartes had better not be following this pattern of argument, or else a great philosopher would really be presenting an absurdity.