Full Idea
Conscious states clearly affect our bodily movements. But surely anything that so produces a material effect must itself be a material state.
Gist of Idea
Consciousness affects bodily movement, so thoughts must be material states
Source
David Papineau (Thinking about Consciousness [2002], Intro §6)
Book Reference
Papineau,David: 'Thinking about Consciousness' [OUP 2004], p.8
A Reaction
This is Papineau's simplest possible statement of what he calls the Causal Argument, which he considers to be a knock-down argument for materialism. I agree, but it is really only an intuition. You never know...