Full Idea
All animals with a liver also have a heart; so we can deduce from this plus the existence of Fido's liver that he also has a heart, but his liver does not explain why he has a heart.
Gist of Idea
Livers always accompany hearts, but they don't explain hearts
Source
Alexander Bird (Philosophy of Science [1998], Ch.2)
Book Reference
Bird,Alexander: 'Philosophy of Science' [UCL Press 2000], p.74
A Reaction
This is a counterexample to Hempel's deductive-nomological view of explanation. It seems a fairly decisive refutation of any attempt to give a simple rule for explaining things. Different types of explanation compete, and there is a subjective element.