 This seemed like a grand unified plan until Saul Kripke (1972) and Hilary Putnam (1975) drew attention to fundamental differences between the metaphysical and epistemic modalities that had tended to be run together throughout this period. They pointed out that, for example, “water is H2O” might well be necessarily true, but not knowable a priori, and “The meter stick in Paris is one meter long” might be knowable a priori but not be necessarily true (that very stick might have been broken and never used for measurements; see A Priori Justification and Knowledge).