A posteriori necessity

Here is an overview of the argument: :(P1) 'Hesperus' is a proper name that refers to the evening star. 'Phosphorus' is also a proper name and it refers to the morning star. But the evening star and the morning star are the same planetary body (Venus). So both names designate Venus. :(P2) If both names designate rigidly, they designate the same object (Venus) in every possible world in which it exists. Therefore (by the definition of 'necessary') 'Hesperus = Phosphorus' is necessarily true. If it is the case that in all possible worlds the identity claim "Hesperus is Phosphorus" is true, the statement is necessary. :(P3) The fact that Hesperus is Phosphorus was discovered by empirical observation, and it could not have been discovered a priori. Therefore, it is known a posteriori. :(C) Therefore, it is possible for knowledge obtained a posteriori to be necessary. Other instances of a posteriori necessary truths include: " is water". == Significance ==

Naming and Necessity is among the most important philosophical works of the 20th century. A posteriori necessity existing would make the distinction between a prioricity, analyticity, and necessity harder to discern because they were previously thought to be largely separated from the a posteriori, the synthetic, and the contingent. Hilary Putnam comments on the significance of Kripke's counter-examples: "Since Kant there has been a big split between philosophers who thought that all necessary truths were analytic and philosophers who thought that some necessary truths were synthetic a priori. But none of these philosophers thought that a (metaphysically) necessary truth could fail to be a priori." == See also ==