Full Idea
Shadows are distinct from the physical objects casting the shadows and irreducible to them; any attempt at reduction would be incoherent, as it would entail identifying a shadow with the object of which it is a shadow.
Gist of Idea
Shadows are supervenient on their objects, but not reducible
Source
Keith T. Maslin (Introduction to the Philosophy of Mind [2001], 6.3)
Book Reference
Maslin,Keith: 'An Introduction to the Philosophy of Mind' [Polity 2001], p.167
A Reaction
Another failure to find a decent analogy for what is claimed in property dualism. A 'shadow' is a reification of the abstract concept of an absence of light. Objects lose their shadows at dusk, but the object itself doesn't change.