Full Idea
The question about Hempel's Paradox is whether contraposition is not only equivalent in truth, but equivalent tout court. It forcibly inserts new predicates into a context of properties known to be connected by nature.
Clarification
A→B so ¬B→¬A is contraposition
Gist of Idea
Contraposition may be equivalent in truth, but not true in nature, because of irrelevant predicates
Source
Harré,R./Madden,E.H. (Causal Powers [1975], 7.I)
Book Reference
Harré,R/Madden,E.H.: 'Causal Powers: A Theory of Natural Necessity' [Blackwell 1975], p.123
A Reaction
[compressed] This seems to capture quite nicely the intuition most people have (which makes it a 'paradox') that the equivalent predicate is irrelevant to the immediate enquiry. The paradox is good because it forces the present explanation.