Full Idea
The main obstacle to mind-body reduction is qualia.
Gist of Idea
Reductionism gets stuck with qualia
Source
Jaegwon Kim (Philosophy of Mind [1996], p.236)
Book Reference
Kim,Jaegwon: 'Philosophy of Mind' [Westview 1998], p.236
A Reaction
Personally I am also impressed by Leibniz's Mill (Idea 2109). No microscope could ever reveal the contents of thought. How can it be so vivid for the owner, but totally undetectable to an observer?
Related Idea
Idea 2109 Increase a conscious machine to the size of a mill - you still won't see perceptions in it [Leibniz]