Full Idea
Many effects that we attribute to conscious causes have full physical causes. But it would be absurd to suppose that these effects are caused twice over. So the conscious causes must be identical to some part of those physical causes.
Gist of Idea
It is absurd to think that physical effects are caused twice, so conscious causes must be physical
Source
David Papineau (Thinking about Consciousness [2002], 1.2)
Book Reference
Papineau,David: 'Thinking about Consciousness' [OUP 2004], p.17
A Reaction
[Papineau labelled this the Causal Argument] Of course two causes can combine to produce an effect, and there can be redundant physical overcausation, but in general I think this is a good argument.