Full Idea
The categories definitive of a given science mark off boundaries that are largely invisible within science at lower levels. That is why there is, in general, no prospect of reducing a higher-level science to a science at some lower level.
Gist of Idea
Higher-level sciences cannot be reduced, because their concepts mark boundaries invisible at lower levels
Source
John Heil (Philosophy of Mind [1998], Ch.4)
Book Reference
Heil,John: 'Philosophy of Mind' [Routledge 1998], p.116
A Reaction
This sounds slick, but I am unconvinced. Molecules only exist at the level of chemistry, but they are built up out of physics, and the 'boundaries' could be explained in physics, if you had the knowledge and patience.