Full Idea
The 'Principle of Reducibility' says if an object is a system of objects, then every property of the object must consist in the fact that its constituents have such and such qualities and such and such relations
Gist of Idea
Reduction requires that an object's properties consist of its constituents' properties and relations
Source
Wilfrid Sellars (Philosophy and Scientific Image of Man [1962], p.27), quoted by William Lycan - Consciousness
Book Reference
Lycan,William G.: 'Consciousness' [MIT 1995], p.95
A Reaction
This sounds to me a more promising attitude to reduction than all this talk of Ernest Nagel's 'Bridge Laws'. If we ask HOW a higher level property arises because of a lower level property, we can describe a mechanism rather than a law.