Philosophical
Goodman's Grue Paradox
All emeralds observed before 2050 are 'grue' (green before 2050, blue after)
Overview
A paradox about induction and projectible predicates.
Define 'grue' as green before 2050, blue after. All observed emeralds are grue (they're green before 2050). By induction, all emeralds are grue - so they'll be blue after 2050! Why is 'green' a legitimate projection but 'grue' isn't?