 Philosophers generally agree that we can be justified in believing  15bi  and  15biii  only empirically. Yet many believe that they are necessarily true. If they are necessarily true, that would mean that it’s false that we can be justified in believing necessary truths only a priori. But some philosophers think that  15bi  and  15biii  are not necessary truths and the relevant necessary truth that we can know a priori is that the essence of any type of matter is given by the basic elements of which it is composed. On this view, in the case of water and rubies, we discover empirically what those basic elements are and the propositions that state those discoveries are contingent. Philosophers who hold that  15bi  and  15biii  do express necessary truths usually think that there are descriptions that fix the reference of the terms, for example, for water, the description, 