Modernist movements believed their own movement could replace all others, that there was no question of their perfection, and no question of their progress.
Postmodernism, on the other hand, believed they owed their existence to Modernism, that perfection was impossible, and they were no better than or worse than the Modernism that came before them.
While Modernism believed it was internally consistent and readily self-definable, Postmodernism saw itself as play and a system of difference.